The is a lake called Canyon lake a few miles north of SA and I've caught a few crappie while bass fishing in it. I don't think it is good but it would at least give you a place to go.

Another lake 75 miles south of San Antonio called Choke Canyon. I think the crappie fishing is decent there.

I have never fished for crappie but I think next time I go out I just might give it a go.