I guess the answer to your question is what is your budget? It sounds as tho you may be new to the sport, so probably an aluminum-framed bike with some decent components, which will set you back less than $2K. I currently ride a Jamis Supernova with a 51.5 cm top tube, but there are plenty of great smaller bikes to choose from out there. If there's a LBS in your area, that's a good start. You might want to check out Kona's Jake the Snake, and Scott's CX Team. Both those bikes have 52.0 cm top tubes which should fit you, both are a good value, and both have been favorably reviewed. By the way, the key measurement for finding the right size is the top tube. Find someone knowledgeable who can size you into the right bike.
I am new to the sport and currently race on a mtb bike. As for budget, <$2k sounds about right. I'm guessing I'd pay more for a great fit. I have a custom road bike, and the fit makes a huge difference.
Why is tough for me to find a bike? I have a normal short guy torso, but really short legs. One LBS already told me to go custom. I did check out the Kona Jake the Snake (47) and Specialzed Crux (49) which were both way too tall.
I'm guessing the Cannondale CaaD X (45) or Specialized Crux (46) might work, but it would be close in stand-over height.
Tsutaoka, that's a tuff one -- sounds as though a frame with a sloping top tube may be the answer but I'm hesitant to advise you, as I'm not really an expert regarding bike fit. Do be careful of toe overlap on smaller frames -- which is more an issue with slow-speed 'Cross riding than Road. Good luck.
Here's an option - In addition to extremely well designed smaller bikes like the Redline, you could always consider converting your MTB for more CX specific use. As you probably saw last Sunday, many Bay area courses put the advantage to the MTB.
With 26" wheels, rigid fork (optional), skinny tires (set up tubeless with Stan's, of course), a 2x10 setup, and using flat or drop bars would work great. Lower standover height, geometry set up for tighter slower speed handling, and only a potential small (but immaterial) weight penalty.
For now, since you're already using your MTB, save a thousand bucks and get a CX wheelset to swap for races. Get a tighter cog in the rear for better gear ratios, and don't forget to use an easily swapped chain (SRAM powerlink or similar design) that is ~4 links shorter to match the smaller cogset in the rear. You can get a killer light wheelset and chain from your LBS or Ebay for a few hundred bucks.
Then, take your time with a new full CX bike, whatever direction you go. Look around, really ride a few, and then commit.