Buying a bike from a distributor like Bikesdirect seems to be a very cost effective method that many people are satisfied with, however if you enjoy mechanical endevors converting a road bike to a single speed can be a satisfying and simple task, depending on the bike of coarse.
If you decide to convert your own I suggest familiarizing yourself with Sheldon Brown (www.sheldonbrown.com) who was a prolific mechanic who assembled an amazing database of bicycle knowlege, and he happened to be quite the fan of fixed gear conversions (which applies widly to single speeds) his website covers almost every scenario imaginable and he is highly regarded among the community (The United Bicycle Institute references his material in their coarse work).
As to buying from BikesDirect, they have several SS bikes for 299 although if your willing to pay a bit more (350) the Kilo TT seems to be an excellent option, I don't ride one but it has legions of fans (que Scrodzilla)