About the biggest 15" that will fit without lifting the suspension or clearancing the fenders will be a 31x10.50x.15. A 32x11.50x15 might clear the front, but the rear may need some trimming or a slight lift to clear the fender lip. Both sizes will be fine on 8" width wheels.
You mention inexpensive, you're not going to find that in a good truck tire these days.
