Dev hardware officially tends to remain property of the console maker so that gets tricky at some level (we will skip tooling up for a raid on
https://www.dualshockers.com/nintendo-unveils-new-developer-portal-site-replacing-warioworld-com/ ). I don't know if it will have been returned compared to when it was current (and if DSi stuff is on your list of things to find that was is that was still comparatively recently they were still giving it out to new devs as a test) but it is a concern.
Four locations where you will find it with any kind of reliability -- we all hear stories of good fortune in yard sales, estate sales, electronics recyclers (might be worth putting your head round the door there but not like they don't know all this and either don't care, don't want to risk the wrath or have means of getting the money for it themselves) and whatever but even if I was commissioned to find such a thing and thus putting in high effort I doubt I would include those in my searches (might run any listed estate sales through the list, someone's uncle will statistically have to have worked for Nintendo eventually, but even that is a bit more effort than is likely to yield much for this rather than general game dev related stuff).
1) Game developers
2) Game testers/publishers/those affiliated with development that might need to view builds from 1) and not be in house for them. This can include trade shows (see story of the Sony SNES addon) but that is more for current or soon to be released stuff.
3) Suitably blessed reviewers (think ign, gamespot, kotaku and places on my little list at the end of
https://gbatemp.net/threads/the-201...-troubled-media-collection-sold-again.535563/ ). This can also be a tiered thing as some lesser trusted puppets might not get true dev grade stuff able to run dev code and instead just be late stage testing, or might have video capture options instead (granted what I saw for the DS was not much for even gamespot I think I was watching where it was basically a shroud, mirror and camera).
3a) Sometimes the kiosks at shops might have something more non standard but that is usually not dev hardware as much as custom secure stand, maybe in the case of the DS with someone tapping lines for a controller (think rapid fire) if it is not just a DS in a steel clamp on a springy thing. Kiosk stuff for the DS was also more download play based and used a DS itself. If is going to be dev hardware that is more travelling trade show -- see some of the stuff where they would turn up in a best buy or something during E3 in random states that were not California to give people a chance to play which could be contracted out but more likely was strictly monitored and returned.
4) Those on places like here, emulator forums, forums more into development, exotic hardware and such like. I don't know where I would particularly look these days (both Nintendo focused and general would be places to go), though wherever the peeps from the old assemblergames found themselves would be a start.
Bankruptcy (you have been doing this long enough to know how volatile the industry is), clearouts (sometimes moving out of an office old stuff gets left behind) and stolen devices then being where you find them.
To this end get involved in the bankruptcy scene (frequently still in person auctions* or archaic websites) and look for game devs and the like. This also means living in the US is good but you probably want to be in California, Washington state (also allows you to maybe jump across to Canada though whether Vancouver does all that much for games these days vs Toronto and even Quebec was not unknown here I don't know), New York (which also grants DC and maybe Toronto or Montreal from before), Texas maybe (will have to check but not so many Nintendo handheld devs there whenever I looked into companies making the releases I was posting, and its little tech and entertainment boom happened somewhat later in the day there).
On the flip side if you do live in one of the states (or parts thereof) where a mortal coming up might afford a house one day there were still devs that did things there from time to time (three months rent on a dusty corridor in Redmond or San Diego might be several years rent or deposit on a mansion in does not touch sea or Canada places) and you are far less likely to encounter turbo nerds that know all I have said in this post, and might also want whatever ephemera to go along with it (if they had a nice statue of their lead character from when times were not so tough/that VC or pub money was flowing...) such that the auctioneer prefers to deal with them rather than your specifics.
I will also note if the average dev lifespan is what 7 years or something we are now rather late in the day for most things here for matters DS. Not impossible, especially in the smaller states if something went one man band/basically continues on paper as some do, but you tend to either fail, make it to a sequel and then fail or get bought by EA with all that it implies. To that end best to also check if they or an immediate predecessor that is basically a rebrand (
https://www.mobygames.com/company/crystal-dynamics-inc if you wanted a site for such hobbies) were active on the DS/DSi.
*do check the skips afterwards or have a word with whatever auctioneer is handling it as whatever did not sell might well be going in them, and if you turn up to find it for those more interested in doing the office fittings, or someone buying it contents and all, then yeah.
Sometimes it will be a knowing sale, other times those dragged in to auction the contents off of a failed dev or clear the building following a sale where the good stuff gets dragged off by the owners and furniture and fittings gets left sees them find a shiny bauble they sell on as a normal item. To that end have your watchlists ready for whatever suitable terms you like on ebay et al and know what they look like (certain colours, markings, non standard ports) -- you won't be the only one looking and if they make it to the front page of some news site on a slow news day the price will go up or item get pulled so yeah.
Equally someone that worked there might wander off with a souvenir and get sold on due to money, divorce, death or whatever.
Going further 4) is a rather circular community and someone that is also doing the money, space, death, divorce... thing may well have their good fortune from one of the things above be wished to be sold on.
Demand for such a thing varies as it can also influence what is done -- original xbox dev hardware will likely be in demand for years and years to come as the extra RAM and whatnot is well known by the homebrew community and used for all manner of things (such that dev hardware is recreated all the time
https://gbatemp.net/threads/xbox-og-1-6-ram-upgrade-pics.617390/ ). If new games were coming out then that also matters. Here new games and whatever extras are available nobody cares about, and indeed devs over the years came here to find out how to kit out their team with flash carts rather than paying the crazy sums Nintendo asked for individual kits (hard if you have 5 programmers, bunch of artists, bunch of testers and more all wanting a go and not necessarily needing debug info** as much as ability to play code in production) so you might even go to an auction only to score a bunch of R4 clones.
**granted cheat options, savestates and more would put it above some dev kit functionality. Some anti piracy might have to be bypassed but you could also have compiled without it.
