I live in Austin so I don't have a answer to your question - but I was recently looking into vermiculite as recommended by the same book.

In case you're not aware, I did find out that vermiculite has some sustainability issues associated with it - it's only mined in a few places and vermiculite from some mines has asbestos. I understand it's also pricey but haven't bought it myself.

The Natural Gardner outside of Austin (worth a visit if you're in the area) has four "square foot garden" demonstration boxes going. One with "Mel's Mix" and three other soil mixes they sell (rose garden mix, "hill country mix", etc). I spoke with one of the gardeners there, and he mentioned there wasn't a significant difference in the vegetable yield between them.

I've decided just to focus on making sure the soil in my garden is rich with organic materials and fertilizer - without the vermiculite.

Not pushing you away from vermiculite, just passing along info.

I think Marshall Grain on East Lancaster in Fort Worth carries it. Might check before you go.

I second the organic materials and fertilizer mixes. I've been working mulched leves into the soil for some years now and buying bags of the cheap compost manure and it works wonders. I get the compost manure at Home Depot for just over a buck a piece. It's always at the back of the garden center next to top soil, but top soil is not the same as the compost.

I got some coarse vermiculite on eBay. It wasn't cheap. I didn't realize that there was a sustainability issue with it though. I wonder if expanded shale has the same issues. The shale seems to have the same properties as vermiculite. Marshall Grain's website says they sell it AND it cheaper than the vermiculite!

Here is a link that might be useful: Marshall Grain