You might receive the most rapid responses from the Square Foot folks
http://forums.gardenweb.com/forums/sqfoot/

Here is a link that might be useful: Square Foot folks

You are off to a good start.
About the vermiculite/perlite, use the web to search your county for store that sale them.
I found the biggest "Brown Turkey Fig tree" at the lowest price by checking the 3 closes counties to my house.
Their were 8 nurseries & some of them were listing in all three counties.
"various bags of" some of the bags at the big box store are Okay....maybe good, but you never know what you are getting & if it is composted. So stay with what works for you & start a grass clipping/ leaf compost pile as soon as you can. I am all for double dug beds or loosening the soil a few inches, before putting up walls to hold the compost mix.
Love to see photos.
Good Luck.

All compost is organic. Since compost is made of vegetative waste, maybe animal manures, and that is organic material that means the end result, compost, is organic.
Keep in mind that Bartholomew spent much time finding what worked best, although something similar to "Mels Mix" will work. Also be aware that the term "Topsoil" means nothing because "topsoil" is nothing more than the top 4 to 6 inches of soil. What you need to know is what is in the "Topsoil". Potting soil is nothing more than either peat moss or finely ground bark, there is, most often, no soil in potting soil.

Here's what I'd do:

Fill your bed with the cheaper stuff and save the "better" stuff for your root ball areas.

Also, a lot of the potting soils out there already contain perlite and vermiculite.

I've always subscribed to the saying that the best thing for good garden results was the gardener's shadow. In other words, Mel's mix isn't magic. Lots of folks have great results with their normal dirt, wheras lots of folks struggle to produce much of anything with Mel's mix (the growing medium is only part of the equation).