Maisto makes some nice Plastic ones and Jada makes some great ones as well but you will have to buy the whole set to get the trailers. Not bad in price.... Maisto's are $6.99 and Jada are $10-12 since Jada puts more diecast in there Sets.
Looking for trailers only, hate to buy a set with a truck and a car or whatever just to get the trailer but I may have to. I know that brand "Speed Wheels" Walgreens carries used to have the flatbed trailers in the mix. I used to find one about every 3rd or 4th store I stopped at. Those trailers I think was like .89 cents each or so.
Look to farm toy dealers. Ertl made a flatbed trailer to tow behind the pickups that will hold one car nicely. Many of these have a pair of wheels on either side and a rear drop down ramp. Ertl also made another trailer before this that has 3 wheels on each side and was intended to haul long implements but looks good hauling cars. Also look at a web site called Moore's farm toys. They have some trailers made to haul as well.
Good Luck and welcome to the frustration!!! As simple as this is and the amount that would sell, its hard to believe any diecast company product development manager that knows anything about their job has not pulled the trigger yet ( i believe its then having to design hitches for the diecast vehicles they make is the main holdup).
Its a NO BRAINER there are no licensing fees as it can be a generic trailer or a trailer with the diecast company name on it. Very expandible, make the trailer & make it with a tool box at the front a tire rack or tire rack with toolbox and make those parts interchange. You can make 1 car trailers, 2 car trailers, and so on, open trailers, enclosed trailers, race trailer with living qtrs, and each of those can be made in either or better yet both bumper hitch trailers & gooseneck.
Its a CASH COW that nobody has stepped up to slaughter yet and to figure out why not is so frustrating its make you want to pull your hair out!!!!
Ya Jada and maisto make trailers but they are CORNY and way out of proportion, they are NOT A 64th scale trailer (measure and do the math). Winners circle makes the best one but you have to buy a set for $12.99 to get the trailer!!!!