Small house trailers are available through Tumbleweed Tiny House Company. Many people also use flatbed trailers that are widely available at Lowes, Home Depot or Northern Tool.
Tiny house trailers come in several sizes. It is important to make sure that the trailer weight limit is sufficient for the house that is to be built on it. Trailers can sometimes be purchased via Craigslist.org, local classifieds and Facebook pages dedicated to small houses on wheels, and used trailers can be purchased at a substantial discount. Recreational vehicle retailers sometimes carry trailers as well, or are able to special order them for customers.