While I don't know your specific ailment, mixing shouldn't be much harder than putting down a pre-mixed bag. Just put them down in layers (think lasagna). You'll find the compost and peat moss in any box store. I found a great store in Houston for the vermiculite:

Vermiculite Products Inc.
3025 Maxroy St
Houston, TX 77008
(713) 869-6663
Kind of near Memorial area
$8 for a 4c.f. bag (grade 3, though, so almost as coarse as 4)
Note that they only accept cash or cheques. No credit cards.

Here is a link that might be useful: More info at this link