I remember seeing .25's on occasion, because I was thinking to myself, "is there really such a thing as a .25 cal pellet rifle?". I know now, but didn't then. Never saw a 25 cal rifle in Walmart though.
.177 and .22 are still findable, but the selection is very small any more. Mostly Crossman, with the very occasional Gamo Rockets, which isn't horrible, at least for my .22. I always liked the Rockets for larger critters, but find the Crossman hard enough to penetrate pretty much what I need them to.