Amazon offers Prime members Officemate Steel Thumb Tacks 100-Count for 71 cents with free shipping. (Non-members can spend $25 or more to qualify for free shipping; otherwise, shipping costs $5.58.) That's the lowest total price we could find by $3. These steel tacks have a 3/8" head.

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.