promix seems to be rare down here in houston too. ive found 2 places so far that stock it... but at 40 bucks a bail. id call your local hydroponics stores as well as the feed n' seed/farm supply places if you haven't already.
I was trying to find some as for this site is loaded with people saying to use it however after weeks of trying to finally emailed the rep and turns out the closest to me is 1 1/2 hrs away so no pro-mix for me u can find yours HERE then click on contact us then on the right click on find a distributor then pick texas.
good luck
edit to add this
Harvest Supply
2317 Cullen St.
Ft. Worth TX 76107
Ph.: (817)335-2601
Fax: (817)338-1045
i found that when i looked