In our area, the only class with a spec tire right now is Spec Slash which is a very hit/miss class... completely dead on turf and sporadic turnout on dirt.

The Slash treads perform well enough for turf and last for many months of use, at least for slower speeds so anything that has some sort of pin will work, I'm thinking a thick heavy block/pin medium compound that is designed for a RTR should do well, pick any brand that sells buggy tires separately and that should work fine, maybe something like these HSP tires perhaps?