Didn't catch where you are, but they are out there. I'm in a similar boat without the size restriction. I'm looking for a. Al framed, CF forked, 105 equipped 54cm CX bike. Oh, and I want it for $500 or so. I've come close, just missing on size. Craigslist is the best bet so far, Ebay bikes end up going for higher prices, can't be tried out and shipping adds a lot.
Right now I know of an Ultegra bike in a 61 for $500 and recently have seen 52's and 56's meeting my specs go for $500-600; even a 56 that went for $300 in a hurry.
Most of these will be 3-8 years or so old. Some of the Tiagra/Sora equipped bikes go for the sub $500 range.
Cross season just ended, so hopefully people will be getting rid of bikes and we'll both have new rides soon.