search craigslist.org. You can search all their sites by typing "catamaran trailer site:craigslist.org" or "world cat trailer site:craigslist.org" in the search engine. If you don't need the trailer right away, search craigslist once a week.
Unfortunately there are not too many used catamaran trailers out there.
As far as new trailers go, don't go too cheap. A new trailer (aluminum) with good brakes will probably run you ~$5000 or more.
A lot of the guys are happy with float on:
http://www.floaton.com/