These can both be purchased from store.Nintendo under 3DS|2DS replacement parts
2DS and 3DS: 1300mAh
New 3DS XL and 3DS XL: 1750mAh
Unfortunately the "New 3DS" uses a 1400mAh battery. I have no idea if either of the other batteries will fit it.
ebay /itm/184018674283 I bought one of these...