If I were you, I'd steer clear of towable trailers for several reasons. First, I've not seen any pull behind trailers that have ever looked comfortable for adults. Secondly, you suddenly may find yourself included in litigation because the person who hired your trailer hadn't properly maintained his or her bike. Here's a website to start at--their pedicab prices are a bit more reasonable than some manufacturers based in the US, so I suspect their products are imported.
http://www.pedicab.com/shop-pedicab-accessories.html If you're interested in domestically built pedicabs, there's one not too far from me in Montana that builds a heavy-duty one using a recumbent trike, but it's not cheap (over $5,000).
http://www.lightfootcycles.com/pedicab.php Before you go too far with monetary commitments, sit down with a lawyer and work out what the details, including contract language for future operators. Also, meet with an insurance agent to get a feel for what coverage will cost since you'll want to include that kind of data in your business plan. Good luck.