I mix mine from local outlets. Napa 8822 from Napa autoparts (verifying that it is in fact Diatomecious earth rather than clay), and then pumice and scoria from local landscape supply. I like to mix theses 1:1:1 by volume. It seems to work well for me and my needs. Your mileage can and will vary. There is alote written on the subject of soils and lot of arguements too, so experimenting is good.
If you haven't already, look under the resources tab for the site and read the article on inorganic components and the one on soil physics. Together they explain a lot about what is needed in a mix, and which components do what.
Failing that, do a search for Booone's mix and either buy a bag or build it youself. It is also a solid choice.
These are just my takes. One of thousands of opinions on the subject, and just as apt to change as I learn more over time.