50's and 52's should be easy enough to find, might not be track specific but they are definitely out there online in all the common BCD's.

I'm running 50/20 right now. Yeah, I know, it's a weird ratio - it's mostly a result of what I had lying around when my old cranks gave out. Kinda sucks because there are only 2 skid patches and I have to spin like a madman if I'm trying to keep up with somebody, but it works for the stop and go kind of riding I'm usually doing. In my opinion, people who do city riding generally over-gear their bikes, so 50/20 doesn't bother me too much.