I would take a look at Walmart or other box stores. A tire dealer might have some tires that are to worn for a vehicle but still have lots of life left in for a wagon and probably give or sell for cheap to you.
Case-IH Farmall 45A, Kubota M8540 Narrow, New Holland TN 65, Bobcat 331, Ford 1920, 1952 John Deere M, Allis Chalmers B, Bombardier Traxter XT, Massey Harris 81RC and a John Deere 3300 combine, Cub Cadet GT1554
I buy all my tires from a local tire shop. A good floatation tire isn't cheap but I like them a lot better then used truck tires. Price wise my tire shop is usually within a couple dollars of Wal-Mart so I just keep dealing with him and get great service. Quality Farm and Fleet use to sell farm tires like that so you may be able to find a local farm store and see if they have them as well but I still think a decent tire shop will be your best option.