The best place to buy large tires is close to home to lessen/eliminate shipping cost to the end user.
You don't post any kind of location info, so you probably aren't going to get many useful answers.
I've bought several tractor/skid steer tires over the years and got em from a local truck/car tire shop. Called a few places and soon I had much better prices than I'd found online. I was surprised that the shop was able to help me out.. just a small tire place, didn't think I do so good cost wise either.
IH Farmall 656 gas/ IH 240 Utility/ 2, Super C Farmalls/ 2, Farmall A's/ Farmall BN/McCormick-Deering OS-6/McCormick-Deering O-4/ '36 Farmall F-12/ 480 Case hoe. '65 Ford 2000 3 cyl., 4 spd. w/3 spd Aux. Trans
Check here, and see if they have anything your size. This is the link to their hot deals page, but there are many other sizes.