Which 25-20?
There are two. The 25-20 Single Shot, is a long narrow case, and a bit tough to find, though I think Starline has made it recently. Mainly found in older single shot rifles.
The 25-20 Winchester is not super common, but you an use 32-20 or 218 Bee brass if you cannot find the stuff with the correct headstamp. This is what you can get at WSS. Also found in single shot rifles, but in some lever guns as well.
Cheers
Trev