Who's Game house on main street is the only place I know of in Rapid City, I Used to be stationed at Ellsworth and played a lot of games there. Decent tables, decent terrain. It was kind of hard to find players unless you pre made a game before you got there. But there are people that play. Here is their Facebook.
https://www.facebook.com/WhosGameHouse
EDIT Looks like they are having an Apocalypse match on the 29th of June