Any type of porous rock should work but you shhould always match rour rock whith the tree. keep in mind the are going to spend a lifetime together so choose wisely.

Rojo, I have seen "lava rock" at a large nursery and stone supply store, the kind of place that sells fieldstone and slate in big wire containers for landscapers. (It was at Pike's Family Nursery in Atlanta but they don't have it on their website.) The rocks were being sold individually as planters for about $30. Possibly, you would have better luck in the spring.