Pumice is a great substrate and is part of the very popular Boons Mix (Pumice, Akadama, Lava). The benefit of adding Pumice into your soil mix is to ensure good drainage while maintaining a little moisture and really assisting in airing the roots.
Pumice is a type of volcanic rock that is widely mined in North America, used for construction materials such as concrete and concrete block. The smaller particles are used for personal hygiene abrasives, stone washing jeans and many other uses.
Oh yes, and really good to mix into a healthy bonsai soil mix.
Pumice is widely available at construction supply companies, but since you would not need more than 1/4 to 1/2 a pound for a decent size tree. We're happy to offer sifted clean pumice in 1 pound bags.