I've given up. Even when I've gone to a store that always has it, they were out when I got there. I make my own by freezing 1 gal jugs of water.
I open them and spill out 4 oz or so to allow for expansion. Best to let it freeze for a week or more. It will get down to 32F in a day or so, but takes a lot longer to get down to the temp of the freezer which is usually about zero. Leave it sealed in the plastic jugs and you have the bonus of nice fresh water if needed. Square jugs are more efficient for packing in a cooler.
Don't Wanna Die Askin' for Another Chance