Go to
http://www.lytron.com/standard/exchangers.asp. They have lots of radiators in all sizes, all pretty thin. I recommend a 6105 or 6110 with a low profile 120mm fan. Pretty expensive new ($112 and $118), but they are top-of-the-line radiators that far surpass anything comparable. Alternatively, you could purchase a similar heat exchanger made by Thermatron, like this one:
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=1780188858 . Good luck out-bidding all the ProCoolers who've seen it now, though. That Thermatron is a lot thicker, but if you can fit it, go for it.
PS: I have a similar project in the works, just waiting for an mATX nForce 2.