Finding a 24' enclosed car trailer with 5200-pound axles is rare. The 3500-pounders are the most commonly-delivered one (just about every one you see will be 3500). If/when you find one with 5200-pounders, it will be significantly more expensive.
28-footers, on the other hand, typically come with 5200#. But, as you've no doubt seen, they're noticeably more expensive.
An alternative is to buy a 24-footer in good shape and swap out the axles. 5200# torsion axles assemblies can be found for ~$500 each, and you'll need new brakes and wheels/tires. Yup, it'll require some work, but it may be faster than waiting for one already equipped to show up.
On the other hand, what are you planning on towing? Judicious use of a 24-footer with 7000# gross can be done easily, unless you're planning on towing a 4-wheeler or something like that also. Ask yourself if you're actually planning on adding in over 700 pounds of tools, equipment, parts, gear...if so, you'll probably want the space of a 28-footer anyway. - GA