Those wheels are pretty common, I think they're a 14.5". If you look a little you can find 8-ply tires in that size.
If you're going to unhook it with heavy stuff, a landing gear from a semi trailer makes a heck of a tongue jack, just be sure to get the one with the gearbox, got two speeds there.
All the Amish who run sawmills in my area will deck a trailer pretty cheap.. might look into it if you have any where you are. White oak with some motor oil for preservative is my favorite deck material. Wood is nice, stuff gets pretty good traction on wood.