Shop Craigslist, Ebay, and this forum's classified section wheels and tires, and any other 4x4 forum classified section.
You can often find 30 and 31 inch tires for sale cheap with anywhere from 80% to 100% tread on them. I've seen BFG AT with 1000 miles on them for sale half price.
Also, most tire stores have cheap tires if you don't care about brand. Sometimes they have used tires very cheap.
Warning: I often edit my posts a few times to get them complete or correct errors.