It depends on what you are trying to accomplish. If you want new parts, there's Cyclocross World dot com, and your local bike shop as Joseph Davis pointed out. If you're trying to get decent newish parts on the cheap, maybe eBay although I've had spotty luck there - some sellers will throw your brakes in a tyvek envelope with no padding, for example. IF you're working on a budget build, scavenge wherever. 110 bcd cranks aren't a new innovation - they were around for quite a while from Shimano, Sugino, et al. Old touring bikes and hybrid bikes can be had cheap and have cantilever brakes, 700c wheels, and quite functional derailers.
Also, see if there's a local 'cross group. If so, more than likely they'll have a group mailing lists where everyone can chime in and members will post their stuff FS from time to time.
I do a lot of shopping around from my LBS to eBay but 99% of what I do is online, I just bought a SRAM PG 1070 rear Cassette for my Crossbow for 58$ from my LBS off their website it was their deal of the day.
So far all of what I have gotten has been new never ridden, I guess that if I was home more and not out of town as much as I am I would hit Craig's list as well.