Amazon offers its Amazon Prime members the Heat Factory Large Hand / Body Warmers 30-Pack
with free shipping
. (Non-members can spend $25 or more to qualify for free shipping; otherwise, shipping adds $5.58.) That's 30 cents per warmer and the lowest total price we could find by $32. Each warmer measures 5.5x4", and purports to provide over 20 hours of heat.
Amazon Prime membership is $79 per year; however, Prime is free for college students for six months, and free for parents and caregivers for three months. (Note that the Amazon Mom program no longer has a wait list for members.)