It has been awhile so I'm not 100% certain, but I think that I saw a lot of packs that were PVC or some other type of plastic. I only bought one like that. It has white polka dots on a red background except for the front pocket which has a red background with a Minnie Mouse on it. I think I remember that I saw quite a lot of PVC or plastic packs that I was unsure of size-wize. Some were still in packaging and came with more than just the fanny pack. For some reason I decided that they might be really small and not hold enough for my kids. So any that I saw not in packaging that looked similar, I avoided. Even with my youngest I figured that I could do up the straps of a larger one and she could wear it.
The thing that started me looking for them, was reading travel guides almost obsessively and somewhere I had seen a tip from a mom who said she had each member of her party wear a fanny pack and had a small water bottle hanging from a carbiner on the strap. They refilled the water bottles at water fountains and sipped water while waiting in long hot lines. So I bought some small water bottles at Dollar General ($1 each) and bought the fanny packs from ebay. I also got items that I had seen in other tips such as super cheap rain ponchos in a different color than the ones sold at WDW. When we get up each morning I'll check the kids packs and put in some snacks, such as a Cliff bar, Cliff blocks, sunspire peanut sundrops (which are sort of similar to peanut m&ms) and/or a little pack of crackers.
With my crew I really need to avoid the temptation to let them indulge in all of the candy and junkfood that will be available. We generally avoid artificial colors, artificial flavors and certain preservatives and it really helps with ADD type problems and other behavioral and sensory issues. So it takes planning to do Disney and be ready to eat there. I'm still waiting to hear from someone at WDW about the best way to prepare in advance for eating in the restaurants. I've heard from others who follow the same diet protocols that the Disney restaurants are wonderful to work with, but each person has a different story of the best way to work with them in advance and calling WDW to get any consistent info or advice about planning for dietary needs has been like banging my head against a wall. Atleast our snacks are ready!