AFAIK, there is no .25 rimfire being made. Around 1950, a friend had a .25 Stevens pump gun and even at that time had a hard time getting ammo though I think it was still in the catalog. Not long after, it disappeared, I guess forever.
Some .25 rimfire guns can be lined to fire .22 LR, but trying to do that to a gun like that pump would not be practical.
Jim