Your best bet is to find a little indie shop that specializes in fixed gear builds and/or vintage builds. We have a couple around my area and they're the only shops I'll really buy anything from. The main stream shops I find to be a turn-off... selling more and more expensive new bikes with crappier components than the year before.

A shop that specializes in building up old stuff can probably throw together a fixie out of used parts for fairly cheap. Maybe they won't let you take it home for the evening, but you can buy it and try it out a lot cheaper than if you go buy a new mass-marketed fixie.