Walgreens
You may have to call around to see who has them in-stock as they tend to disappear during the summer travel months.
Personally, I think the old flight attendant trick of paper towels saturated with very warm water (NOT hot water) placed in plastic drinking cups and positioned over the ears is a cheaper and more effective way to equalize air pressure during the climb.
Either that, or get them to chew a large wad of bubble gum (but not to the point of it becoming a choking hazard). The mascination (chewing) causes the ends of the eustacian tubes to open (and equalize air pressure with a "pop").