Go to automobile dealerships or used tire places.
When I worked at Huffines, I got boisking's little brother a matching set of big, off-road tires for his Jeep. Cost: Free
Huffines always has a huge pile of tires on the backside of the dealership by the parts entrance. This is at the Ohio and Plano Parkway location.
There is a guy off of Buckingham and Beltline that sells used tires. His tires are generally $20-$30 a piece with good tread. My dad and I have been going to him for a few years now.