Dave, try to find a landscaping depot that sells lava pebbles in bulk, a bin in the yard where the pebbles are sold by the loader bucket into landscaping companies trailers. Then talk to management and ask if you can sift out of that pile. There's usually enough small stuff in there that its not too big a job. As for pumice I think the only mines in Canada are around Pemberton. A friend of mine had a bulk shipment in to use as substrate for a green roof. I might make a similar order next year, ill let you know.
Jay, if you let the forum know if you're growing trees indoor or out, what species your trying and where you live you'll probably get all sorts of suggestions on mixes you can make yourself.
Mp