Have you tried asking at a local auto body shop?

Another source might be Hemming's.

I took a metalsmithing class and I'm by no-means an expert, but you might be able to do it yourself.

The technique, would be to set the hubcap, dent side down, into hot tar. When the tar hardens you can use a ball peen hammer to gradually eliminate the dent.

Hot Tar? How does one get hot tar?
Heat it in a pan or what.
Sounds interesting I may give it a shot.

Hot tar? Maybe from a roofing supply co. I think it comes in three inch or so paper wrapped cylinders. If you ruin your hubcaps, get some feathers to go with the tar and come looking for jeremy ;)

I'm not sure what tar's flash (or even melting) point is, but you may want to ask the supplier whether you should heat it over a double boiler like for parafin, or if direct heat is OK. You don't need to catch that stuff on fire.

I've got a very stout utility trailer made out of a '68 IH pickup bed with the original Dana 60 axle underneath. Its got those hubcaps, but the center's are cut out for the axle ends. The hubcaps are very rough any more, however.

Rather than the hot tar (oouch) a bag of sand in a good leather bag if possible will work fine. Just gently pound out the dents then using a flat 1-1.5 inch piece of steel gently tap and tap and tap the peened surface till flat time consuming but works (poor mans english wheel). Good luck on your endevor.