Luckily I don't struggle with getting my hands on bonsai soils. I pay through my teeth but at least I know it works. When I can't get my hands on a pre-mix, I tend to get some river sand, perlite, peat moss, fine-ish gravel - I don't like lava rock all that much since I found that it kind of reduces growth rates. Not sure if there is actual research on that. Either way, at least nurseries and garden centers etc are almost never out of stock on these.