Brand new? Might be hard actually and is most likely to be people that bought things to sit on for days like these, or rare case colours and such.
Second hand. Anywhere they sell second hand consumer electronics really. Prices will vary a bit but rock bottom prices are still available if you look hard enough, do also consider buying a bundle from someone (mothers seeing their kids moving onto other things don't have time for parting out all the games so might sell you the console and game bundle such that you can sell on the game bundle individually and make your money back).
Might as well do the quick list of things to check for
Both DS and DS lite can have issues with the game slots so make sure it loads a game. Replacement slots (note they are different) are available but will require some fairly decent soldering ability to replace.
Both also have issues with blown fuses and charging (it will flash the orange light rather than be constant). Easy enough fix (some basic soldering will be needed for the suggested methods) but do note it. Said batteries might also be quite old at this point (2004 for some models of original DS) and said batteries are not entirely trivial to get good replacements for.
The shoulder buttons on both can get a bit hard to activate (missing presses or needing to be pressed hard). The basic repair is to take them apart and clean them (needs the relevant screwdriver), however some opt for replacements (usually from another DS) as a longer term thing.
The original DS quite often has broken hinges (it was not a great design). Replacement shells have been made but are very hard to find nowadays (I struggled to find one in 2014
https://gbatemp.net/threads/ds-original-phat-replacement-case-review-56k-no.364632/ ).
The DS lite's power switch (on the side rather than on top) can be tricky so make sure that works.
Sound problems are incredibly rare but as you will be testing power on anyway make sure the volume slider is good.
Obviously check for scratched screens, broken screens and any case defects. Touch screen should work but you can test that from the menu easily enough in pictochat or something.
There are some further weird and wonderful problems but nothing I would check for there and then. If you have a DS flash cart there are some homebrew programs you can use to run a greater check.
Replacement parts as a whole are going back up in price, if you can even find them. To that end if I needed to do a proper replacement I would probably hunt around for the classic pink DS/DS lite at a flea market/car boot sale/jumble sale/garage sale/... and cannibalise that as those you can still get for cheap.