Hello,
I just found this forum last week, posted a question, and got an answer pronto. So I think I can return the favor here and offer you guys this idea.
I have built some pier fishing carts out of pvc pipe. Being on the gulf coast we want everything to be non-steel.
Anyway, I found a source for lawn mower turf tires 13 x 5.00-6 on orange plastic wheels. Not brittle shiney plastic but the dull tuff plastic. They have 3/4" bore flange plastic bushings. Price is good at $12 from Burdens Surplus Center

Now for bigger wheels for soft sand here is another alternative to the rollezee tires. ATV front sand tires on aluminum wheels. You can buy brand new pair on alum. wheels for $150 delivered via ebay. They are bald except for the center rib. They make smooth ones too but the are more rare.
You can always find 40+ pairs of new and used ones in ebay for less.
They are 22" x 8" on 10"dia. rims. Do an ebay search on "sand tire*" or "dune tire*". If you want even more flotation you could get a pair of used rear paddle tires and cut the paddles off to make them smooth.