Hiya Barb, I got poly white bags on ebay through HTG Supply, which also oddly has them direct from their own website at a considerably higher price. I bought the 50 pack of 10gal for like 50cents a bag. I see they are out of the 50pack size now, but they have 25packs at about 60cents each for 10gal.
Less evaporation was the biggest growing difference between them and the fabrics, a quart to half gallon less of the water soluble ferts daily on plants of comparable type and size. Root development seemed nearly comparable and they were sturdy enough that none ripped or split.
https://www.ebay.com/itm/GROW-BAGS-B...v76VuQdD3mEHbA