Our Western states are full of it. No need to send to Hawaii. Pumice is volcanic, and can be ordered through Grigsby's
Mexico is full of it, Mt. St Helana has loads of it. Do not gather it from a state park. If you come to Calif.I will take you to a spot where you can gather it. I notice you live in Main. Buy quartzite, pumice, sandstone, granite. All volcanic. Any coarse sand will do as well and Chicken grit #5 or larger.
Castus and Succulents grow in decomposed quartzite, decomposed granite, and a lot of rocks. You can find these in streams, or where new houseing developents are being built out in the woods. These plants are not fussy, but they do not like Peat Moss, unless they are jungle growing plants. They only need 10-20% of leaf mold, or bark. Go to any rock and gravel store, deserts and see what you can find, or Home Depot, perhaps you can find a building supply store and get washed coarse washed construction sand.
These are just some suggestions, you are going to have to make the choices. If you have high humidity put in more sand/gravel and no organic matter. Experiemnet to see what soil does the best for your location. Alway check and see whre the plant came from, and I don't mean Wally's , Most cactus will come from North America or South America, they grow on mountain sides, or deserts, so that that into consideration, Succulents come from all over the World, Mexico and all up our west coast. Some come from the mountains of Europe, others from Africa, below the equator, or from Tropical pleces. You will need to know this information before you can make the choice of what they will need. I hope this response will help others on the forum as well as you. Good question. Norma

Thanks, Norma. I'll check out Grigsby's.

PS. I live in Mass, not Maine, although Maine was part of Mass before 1820.