I think charcoal would be fine. Easy to buy and not to mention, light in weight. I am surprised that the Thomson nurseries do not have it. They should be available for orchid grower. I used to grow alot of orchids and I use charcoal and broken bricks. Anyway, you can always buy charcoal for BBQ from provision shops. If you really want fancy stuff, some nurseries sell bark chips for potting or orchids.