Germinator, as you can see from earlier posts in this thread, there is some variation in the way gallons are calculated for these bags. My "12 gallon" Easi-Lift bags with handles hold about 5 real gallons, and so on.
I came across this item in Home Depot online and ordered 1 pack of 25 for $14.97 plus shipping and tax. I haven't seen them yet, so can't vouch for them, but I seem to remember reports from folks who have used these black/white bags with success.
Please let us know if you discover other sources. By the way, if you're interested in what they sell at phoenixorganics.com, including the Easi-Lift grow bags, I suggest you call them. They're friendly and helpful on the phone (and admit that the website isn't reliable in many cases).
Good luck.
Gary
http://www.homedepot.com/h_d1/N-5yc1...&storeId=10051