You may try ebay and Craig's list. Do the craigslistsearch engine. You choose the miles away from you the search is to explore. On ebay just click the "nearest" block. A good used one can save you many $$$$$.
Tommy Trailers and Featherlite are the very best, I think. Wells Cargo (there are several others in the WC category but I can't think of their names right now) is good also.
As to axles I do recommend tandem. As to springs or torsion, I not sure which is best for hauling a car. If you do get an enclosed trailer get it as wide as you can, about 8', and with a door on the left side so you can get in and out without having to crawl over the vehicle.
Good luck on finding a good one at a reasonable price. I recommend staying away from home-made trailers. Some are good but some are not.
Charlie