I've been using aspen as of late. You can pick it up at Tractor Supply at a relatively low cost (I believe it's around ten dollars per bag, and the bags are pretty sizable). I also get cypress mulch for some of mine, but I have to either pay pet store price or buy it at expos. Personally I really hate cypress mulch... just... ugh. It gets in the water dishes every single day and it's way too expensive.