Those of us that have them, rarely sell them. I got mine by enlisting, busting my ass for two years in a line platoon, taking an indoc, earning myself a school seat, suffering through nine weeks of strenuous daily tests to prove myself worthy, and graduating to earn the MOS. After all that, we don't just put our shit on eBay.
Scour the Army/Navy shops in your area for one and keep an eye out on eBay for some of the collectors/dealers who find and sell them. You likely won't find one quickly or cheaply, but they do appear from time to time. Be prepared for a bidding war.