Well, if you don't find someone nearby who has a small amount to give you, or sell to you, go a pool supply store - there must be many of them in Hawaii. As for cost, you may be able to buy less than a 50 lb. bag, but even if you had to do that, in Hawaii, the price should be less than $10. In some places in the U.S. mainland, it's as low as $6.
Check it out.