After thirty years of bonsai and hundreds of pounds of soil, and living where components are easily attainable, I can say there is no substitute for akadama.
But here is the rub.....
Why do you want it? What is it that you feel can be gained by finding a substitute for something there is no substitute for.
I'm having some difficulty sourcing akadama locally. Price isn't the primary concern, though I wouldn't balk at finding a cheaper alternative. So far, all the alternatives I've seen aren't superior based on price.
Expanded shales (haydite and many hydroponic soils are made from shale that is not naturaly porous, or at least not porous enough for our horticulture needs. Heat treating it causes it to expand and "open up" creating a more porous structure.
In hydroponics, they seem to make some distinction between expanded shale and clay-based products. I'm particularly interested in the clay-based products, ...as mentioned previously.
I have said it a million times and still people look at me crazy when I say it, and I will say it again. Akadama works because it is "organic". It is volcanic soil, much like all the premium soils around the globe that are perfect for growing things. there is nothing better for growing than growing in soil created froma volcano. Ever wonder why?
On what basis do you claim akadama to be volcanic? Japan has many different kinds of soil in different regions. Where's akadama (primarily) mined/produced? Or are you saying that Japanese soil exists because it was once spewed from middle-earth? Also, what's your basis for claiming akadama to be organic? All my research into Lava seems to indicate its inorganicity (if that's a word, ...and this browser's spell-check takes an issue with it.) At the temperatures at which lava erupts (and is able to destroy flora/fauna) I would think the flora/fauna would be turned to smoke and released into the air before it would cling to the lava that's burning it. Maybe I'm overthinking. Where can I read more about this?
This group, the Boon followers,
...what would be the one component of Boon mix that can't be done without? You could leave out the pumice and add double lava, leave out the lava and add double pumice. Could you do without the akadama? NO, the akadama is what makes his soil work so well. Leave it out and you might as well be potting in kitty litter. They add nothing to the soil mix except volume. Now you could pot in kitty litter and use fertilizer cakes rich in humates or use a humus tea weekly and get pretty much the same effect without the expense of the akadama.
I notice below that you have Akadama in your soil box. I also notice that there isn't any kitty litter. I've also read that you're a big proponent of humic acid ...and "broke" the news to the (online) bonsai world. I don't think its overstating to say that many folks consider you a pioneer w/ humic acid as it relates to bonsai soil, though you've deferred to the history and experience of the hydroponics/farmers before you as the real pioneers.
Regardless, if you believe what I'm quoting you (in bold) as saying above, why do you deal with the expense of Akadama rather than recreating its properties w/ kitty litter and humic acid?