What level of competition? If you're just looking to have a good time, most of the bars around town are fine for that. The Bar Formerly Known As Jingles (can't recall the new name) down by the field house is a fine place to go if you're looking to play rec league and drink a bunch of beer.
If you're looking to play more competitively, your options dwindle pretty fast as far as sand leagues go. Club Tavern used to have some quality leagues, not sure if they're still good or not. The Coke brothers used to run a sand doubles league on the east side, but this was years ago - dunno if they're still doing it. There used to be (probably still is) a very, very good grass league run by Mike Schwartz during the week - that was always fun, but it's grass, which isn't nearly as good a game as sand.
The best sand games in town were usually down at the Nat, but that's doubles. They did run a rec league there in the summer, but I think you need to be a UW student to participate, plus they didn't have a bar, so what's the point?