2 words.
Plywood
Lumber
A PT Barnum quote comes to mind that more folks should consider before getting on Amazon with a credit card!
Chopped up a bunch of 1-1/8" plywood "pads" out of a half sheet of old form ply I had laying around that was useless for any other household purposes when we got the camper. Down to only about 4 of 'em now and a random assortment of 2x4 to 2x8 chunks for leveling the truck and for jack pads. Best part is if a couple don't make it home, there's more where they came form unless I've burned the scrap lumber recently!