Yes, pumice has a lot of air trapped inside and is volcanic in origin. In terms of function in the garden, I would imagine granite grit works just fine as a substitute. Pumice comes from the SW US and is much more available in CA and AZ compared to the rest of the country. You might look locally for a product called Dry Stall (for horses) which is pumice by another name. Also consider scoria (lava rock) which comes in red or black and is very similar to pumice.
