Build your own from existing bikes. There are articles on how to make anything from anything all over the place. ****, you can make a disk MTB hub work as a fixed gear, and can make a rear derailluer into a chain tensioner. Just ask here, read online, and enjoy. Building your own allows you to become very creative and capable, as well as allowing you to know your bike inside and out. Do what you like, and if you like buying onto the Kool-Aid,

www.fisherbikes.com www.specialized.com www.bianchi.it

Surly, Gunnar, KHS, Spicer Cycles ($$$$$$$$), Colnago, and a small army of botique builders will suit your fancy