I've found that it is now fashionable for most major bike companies to have at least one fixed/ss bike in their 2008 lineup. Most have a flip-flop hub so you can decide whether you want run it fixed or ss depending on your mood/confidence/ability. I have seen a Schwinn Madison, Bianchi Pista, Kona Paddy Wagon, and I know there are heaps more by Cannondale, Masi, Trek, Fuji, KHS, Salsa etc.
I toyed with the idea of building from scratch but unless you already have a good frame with horiz. dropouts or lots of spare parts it would be much easier to order a complete bike.