I've gotten new ones from some of the big box home-improvement joints, the iron ones on my VW engine dolly came from a now-defunct chain. These are supposed to be good for 1000 lbs each. Since they live on a concrete floor, I don't worry about wheel marks. I've seen some large ones at the local True Value, too, solid rubber. Ace had some smaller ones, 3" or so. One of the local tool stores that handles import stuff as well as Dewalt and Milwaukee has a large assortment of iron and rubber-tired castors, all sizes up to 12". Retail prices, though.
Surplus Center in Lincoln, NE had a lot of them in their store, more than made it into their catalog. Just wish they were closer instead of having to limit my trips to when I go through there a couple of times a year.
Stan