We look everywhere last week to buy one. Sam's in Playa didn't have any at that time (don't know about now) we end up buying ours at Sam's in Cancun. They are not the backpack one though, they fold up and have a strap to carry them on the shoulder.
At the time even Sams in Cancun didn't have the backpack style. So I guess your best bet would be Costco in Cancun
Or if you are looking at the camping cylinder shape one, you could find those at Walmart