Sometimes a person will get a great deal on a boat that doesn't have a trailer....or may have a "car topper" boat...and then not be able to do that...so they end up looking for just a trailer. Besides checking with marinas & dealers...you can also consider getting a "kit" and putting it together yourself.
Here's a couple of "kit" boat trailers to consider...
I also found these things on the 'Net...
You could also consider modifying a regular cargo trailer...I bought one of these last year from http://www.shortypen.com/essays/trailers/
This website can search multiple Craigslist locations....although you may have to change an Internet Explorer setting to allow it to work properly.
Life Member David BB Linkmeister
US Army '78-'85 West Central Wisconsin