"Does anyone sell a complete cart somewhere? On ebay I can only find a 369 in 1 cart"
That is walking the line. Much further and you would be asking for a link to a repro which flagrant commercial piracy and thus frowned upon around here.
If however you want a flash cart to fill up then that is your business, and something we know a bit about around here. Most multi carts loaded with games will be terrible (don't know so much about today but classically those ?? in one devices are noted as being 5 games repeated, maybe with a few hacked title screens or cheats), those few that are even vaguely passable will usually be variations on old homebrew designs (
https://web.archive.org/web/20180309112726/http://www.reinerziegler.de/readplus.htm covers a few of them)
With a flash cart you can chose ROMs, make sure you have the best versions, have cheats, have ROM hacks and translations, have homebrew code...
The cream of the crop for GB/GBC flash carts is the everdrive GB X7.
https://gbatemp.net/review/everdrive-gb-x7.664/
That is going to be the closest thing you can get to a thing that allows you to wander around happily with an entire library (or two entire libraries) in your pocket. There were a few attempts to make new flash carts before they came along but everdrive did their usual trick of "let's design a flash cart with modern standards and hardware, not just try to improve on legacy things made when the host devices were still current" and stomped all over those really.
About all you will struggle with is stuff like
http://bgb.bircd.org/pandocs.htm#huc1mbcwithinfraredcontroller or those things which added a bunch of functionality to the device with their own chips (usually stuff like the TV controller, I think there was a personal organizer, and some other random stuff).
Personally I would suggest you cull the library to the good stuff or stuff you find interesting but whole library is something some people like to do. I don't know if there will be some kind of file limit with the device as most usually content themselves with hundreds of games, not the thousands and regions dupes + hacks + whatever out there but you should be able to get far up into it all, or just have multiple SD cards.
If you are doing a GBA then you do also have the option of a GBA flash cart. They should all run Goomba and Goomba Color just fine. While they are not perfect emulators they are very nice, and at the same time you also have the bonus of being able to play GBA games. EZ4, EZ Omega and Everdrive's GBA efforts are the main contenders for a GBA flash cart worth having here, avoid the supercard (bad carts) and fire cards (too small) as well as the EZ 3 in 1 (it is good but needs a DS really).