The last packages of bands I bought were from Walmart. I always use 12. Ten straight back and the last two crossed to keep the first ten on. After use I discard them all. They are too cheap to risk an in-flight failure at under $2 dollars for a 1/4 pound. I don't keep them in a freezer to make them last but I do keep them inside the plastic bag they came in, and also in a cardboard box to keep the unused rubber bands from getting sun damage.

When I first learned how to fly, there were some oldtimers that reused their rubber bands and kept them in a plastic box filled with talcom powder. They said that one or two would occasionally snap, and that was when I decided it wasn't worth it to be frugal there.