Wal-Mart usually puts a large container near the door with large bags of generic ice treats in them. They cost $3 or $4 for an ample supply. I cannot really taste a difference between those and actual Fla-Vor-Ice. But I would suggest trying Sam's Club or Costco for BIG boxes of ice treats.
About a week ago I found Original and Tropical Flavor 100 count boxes at FAMILY DOLLAR. The price was $3.50. That was the best deal I've found. Of course depending on your location and if you have a Family Dollar in your area. Also most boxes have mail-in-rebate on box. =)