Hanes via eBay offer the Champion Women's Vapor Powertrain Shorts in six colors (Black/Magenta pictured) for $5.99 with free shipping. That's the lowest total price we could find by $8. They're available in sizes XS to XXL, although not in all size/color combinations.
Target takes up to 25% off a selection of kids' outerwear and accessories. (Prices are as marked.) Even better, cut $5 off orders of $50 or more via coupon code "TGTGUSYC". Even better, orders of $25 or more qualify for free shipping; otherwise, it starts around $5. (In-store pickup is also available for some items.) Some best bets: