Mario,
Are these newer flat cars that are closer to scale with pup trailers? If so the new ray tractor trailer sets are a good way to go if you can find just the tractors on some of the diecast and auction sites. they made a '61 MAck, and vintage Kenworth and Peterbilt tractors. Also Ertl mad some vintage tractors for individual sale years ago that can still be found at the same places. There was a "50s Chevy Cab over, a "60 chevy, an International and I believ a Peterbilt.
If these are older LTI vintage trailers they are closer to 1/64 scale and Lionel did make separate sale modern tractors. You might also be able to find some of the tractors from the 1/64 NASCAR haulers. for these trailers.
Joe