I try to avoid the word 'bonsai soil' because there is no soil in modern bonsai substrate. I use for ALL plants that are in containes (bonsai, also the most expensive ones, flowers for the house and terrasse, tomatos tec.) 70 to 80 % of baked loam and 20 to 30 % of rough peat. Instead or in addition to baked loam you can take pumice, lava split, hard akadama (not the cheap one, because it decomposes and can kill your trees, crushed bar etc.
Baked loam is sold under many different names. Look for mateirals that are for tennis courts, addtion to soil for golf courses, sports lawns, material used by fire departments to pick up lost oil, material used in the building industry for insulation against cold and heat and for sounds. In America a product called 'turface' is fine. If you ahve a choice between the round particles and crushed ones always take the crushed ones.
In Germany a product called 'Maxit', 'Fibotherm' is OK. Use particle size of 2mm to 4mm. sometimes one gets a wide variation of sizes. Sieve your material and throw away everything that is smaller then 2mm.
Do NOT create a special drainage area in your pot. Your SUBSTRATE IS DRAINAGE SUBSTRATE:
This has the following properties:
easy to get in any garden supply store
one CANNOT OVERWATER! Yes, you can water as much as you want, any fool can water properly
One MUST water often and a lot every time
one must feed on a regular basis, like every 10 days to two weeks (any fertilizer that you can get for NORMAL plants, liquid or not, high nitrogen content is OK!
it is light when dry! This means one can carry very large trees without help.
On many of these products the color is very good. No other top soil needed.
That's it, this is my soil secrete.
2) baked clay or loam
3) substrate ready for planting 80 % baked clay and 20 % rough peat