Check the federal government auction site (gsaauctions.gov) for used FEMA trailers you can bid on. Click the "Trailers, Tractors and Manufactured Housing" link to see if any FEMA trailers are available. Click the FAQ and "About GSA Auctions" links for instructions on how to register and participate in the auctions. Bidding on the site is free. Also, check other sources where FEMA trailers are sold, including online auction sites such as eBay and private auctioneers or dealers.
Test any FEMA trailer you're interested in for dangerous levels of formaldehyde. In 2012, more than 20 FEMA trailer manufacturers agreed to pay a class-action settlement of more than $37 million to Gulf Coast storm victims who said they were exposed to toxic levels of formaldehyde in FEMA trailers. A number of labs around the country sell formaldehyde test kits and will analyze the results for you at a cost ranging from around $40 to more than $125.
Inspect the trailer carefully, in person. Many FEMA trailers have major damage from being used as shelters or from simply sitting outside in a storm region. A certified RV technician should do the inspection if you don't have the expertise required. Be aware that FEMA trailers have no holding tanks for fresh water or sewer waste.
Ask the owner or auctioneer for the trailer's "Useable Travel Trailer Certificate," which by law must be given to any buyer of a FEMA trailer. The certificate makes it clear that FEMA trailers aren't meant to be permanent housing.
Make certain, if you're going to be towing the trailer, that you have a vehicle that's strong enough to do the job. Check the towing and tongue weight limits in your vehicle's owner's manual.