You are near several shops. Out on 42nd and Balboa, there's a small shop called D & D Cycles. On the other side of the park, on 15th and Irving, there's a shop called Roaring Mouse Cycles. Finally, at the east entrance of the park on Stanyan, there are three or four shops, the biggest of which is American Cyclery. I think one of the shop's on Stanyan -- Cyco City, or something like that, may sell used wheelsets. American has the biggest selection of inventory in the area, but aren't cheap. Tons of people selling cheap wheels on Craigslist though. If you want to buy new, there's a Performance Bike at 5th and Brannan in SOMA; that's probably the source for cheapest new wheels in the city...
orbea onix, Cervelo SLC, Specialzed Allez, Cervelo P3 Alu
ditto on the sports basement. If you're looking for group rides, check out City Cycle, on the corner of union & steiner. Their saturday ride leaves at 9, and sunday's at 9:30. Sunday's is a friendlier "no drop" ride but the guys and girls are pretty nice on saturdays too.
I'm on 25th ave and geary. With our proximity to GG park and the GG bridge, we're really in a great place to be roadies.