Is it local or nationwide?
I have a little experience in collecting this kind of data.
www.thebluebook.com is an excellent directory for almost every region. I have always just called them up asked them if I can fax or email some information on my company and go from there.
Having built several homes in a few states I think the concept is great however I warn you to only reccomend GREAT contractors. there are a couple places I would look if I were you .First national websites like http://www.ownerbuilderbook.com/ as a former forum member there I have been contacted by many members looking for contractors.
another place; when people get construction loans they mostly are required to have a service follow the progress, these same services usually walk the clients through the budgeting process, as well as procurring contractors. COBS is a big one along with several others, I`d think they would would be willing to drive customers to you providing you have GREAT contractors.
just a thought