I use Hardwood Stove Pellets from Lowes. Here is a link to the specific ones I use:
https://www.lowes.com/pd/40-lb-Greene-Team-Platinum-Hardwood-Pellet-Fuel/50063465
Oddly, whenever I clean out my bun's litter box (every other day) it smells smoky, kind of like fire or a meat smoker. How weird...