These are the ones I used, they are called Phat Sacks, and they held up well. I only used them for 3 years but they are still usable. I used the 20 gallon for large plants, I don't think they have 15 gallons, but I didn't fill them up all the way, so i probably used the equivalent of 15 gallons. I like having some extra room for watering and to keep the soil in and everything neater, but I did wish they had 15 gallon so I could compare.
https://www.htgsupply.com/?s=phat+sa...ct&dgwt_wcas=1