The size of a travel trailer should be proportional to your needs. A small one may be suitable for a short trip for a small family. Quality is not necessarily dependent on size.
This website has an excellent selection of Lightweight Travel Trailers: http://traveltrailerdeals.com/Travel-Trailers/lightweight-travel-trailers. If you are looking for a second hand one, Ebay and Craigslist are good venues.
A good selection of travel trailers which are for sale can be found on many websites. These websites include "CraigsList" "RV Registry" and also "RV Trader Online".
Ebay does sell travel trailers. They can be found in the automotive section of the website. It is recommended to buy this from someone that has a good seller rating to decrease chances of buying something that isn't as advertised.
When buying used travel trailers you must make sure that everything is in good condition. Also, make sure it has not been involved in any accidents.
Big Tex Trailers and Trailers for Less offer good information about flatbed trailers. You should be able to get a good knowledge base from these websites.
I've actually seen quite a lot of concession trailers for sale on Ebay. If that's no good, you should try www.runwalkjob.com. They have a listing of various trailers as well.
You can buy concession trailers for a good price at: http://www.jrpconcessiontrailers.com/
There are many good websites that offer enclosed trailers for sale. They will be geared towards trailers for sale in various areas. Kijiji is a good website to find items for sale in a community of interest.
Ebay is actually a good source for finding concession trailers in your area.
If you are looking for stores that have used cargo trailers for sale, take a look at Tucker's Trailers and RVs. They have a good and decent selection of cargo trailers that you can buy.
Traileroutlet.com has a wide selection of all types of trailers. Also try trailersforless.com for a good selection of gooseneck flatbed trailers.
I would like to buy some cheap utility trailers. Where is a good place to look for cheap utility trailers in Canada?