Often times it can be hard to find a good bonsai soil mix at your local garden supply store and if they do carry one more often than not it is fairly expensive.
When growing bonsai it is not necessary to use a specialty soil as you can grow healthy bonsai in a basic potting mix however you will see improved results if you do use a soil tailored specifically for bonsai.
If your local garden supply center does not sell bonsai soil they most likely do sell all the components you need to make it at home.
There are three basic things you need to make bonsai soil.
These are potting soil, Akadama and shredded bark.
Akadama is a traditional Japanese additive to bonsai soil.
You may find a similar product called Turface which you can also use.
If you can not find Akadama or Turface there is another option.
It is also possible to use untreated cat litter for this portion of your soil.
If you use cat litter you will want to get one that does not break down.
To test this, soak the cat litter in water for a few days before using it to see if it holds together.
If it turns to mush then it is no good, if not then you should be okay.
Once you have everything you just have to put it together.
Start with the shredded bark, followed by the Akadama and then the potting soil.
Hopefully this gives you an idea at least of where to start when you are looking at creating the perfect soil environment for your bonsai.