Amazon has the 4-Pack Earth's Best Organic Electrolyte Solution (33.8oz bottles; Concord Grape flavor) on sale for $6.73 - 5% off with Subscribe & Save (can cancel anytime) = $6.39 with free shipping on $25+ or with Prime. $1.60 per bottle vs. $5 at most grocery stores.

• Tip: Marketed for kids, but adults in Amazon reviews report using it as well.