if your tractor is 4x4 or MFWD type of tractor. you may need to replace both front and back tires at same time. to keep the gear ratio correct. if you do not keep correct gear ratio, and put tractor into 4x4 or MFWD, you could tear up gears / transmission / etc... due to rear or front tires turning quicker than the other. result is all the force ends up getting placed on things that can tear things up.
as far as finding places locally. normally it takes looking into the old phone book for "automotive" tire replacement, and just start calling 100 different places locally. and be ready to quickly jolt down a name or phone number.
some places do not know, but may know of some place or phone number. always awesome! and when you get someone that handles tractor tires, some may only carry certain brands.
normally suggest calling places up to 2 hours away from you. due to tire prices for
tractors can range a good amount. once you get done with the old phone book, do a search for "ag" or "R1" or R3" tires on internet. to help ya better find places that sell tractor tires.