There are SO MANY good, entry level fixed-gear bikes these days you have numerous options. If you're looking for strictly price, bikesdirect.com is good. If you're looking for brand name, Specialized, Giant, Swobo, Fuji, KHS, and numerous others have bikes.

And if you want to be a little more hands on, you could always look at a conversion.

Now, all that said, if this is your first fixed-gear bike -- and your first bike in general in a long time -- I really wouldn't recommend an online purchase. Go to a bike store and get a bike you can try before you buy. Make sure you go to a bike store that does good fixie/SS business, otherwise you might get bad advice, or steered towards a geared bike (which isn't bad, unless you have your heart set on fixied/SS).

Anyway, yeah, check out a bike store.