I'm looking for a pair of 31" all terrain or mud terrain tires for my rail buggy. I'm looking for a 31 x 10.5 x 15 tire that is good for sand and has a pretty tough side wall (we do mostly trail riding with lots of roots and stumps that jump out at us). What can you guys recommend? Any shops I should check out?

I'd definitely consider used tires if anybody has any.