With orders of $25 or more, Amazon offers Prime members the AmazonBasics Head Strap Camera Mount for GoPro in Black, model no. GP-TD, for $7.49 with free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $6.04.) That's about $8 less than you'd pay for a similar mount purchased elsewhere. It's available in one size fits most and compatible with GoPro Hero, Hero 2, Hero 3, Hero3+, Hero 4, Hero 4 Black, Hero 4 Silver, Hero+ LCD, and Hero 4 Session.
Note that this item is part of Amazon's add-on program and can only be purchased with an order of $25 or more.