I wish I had another Mixamp lying around haha. I would love to have another 5.8. Have you tried Craigslist? Oddly enough people have ASTROs for sale and trade on there.
Astro A40 2013 Edition in Black with Mixamp Pro - Astro Scout Backpack - Hauppauge HD PVR2 Gaming Edition - Gears of War 3 Limited Edition Xbox 360 - PS3 Slim - XBL GT: JoKeRs FuRy - PSN ID: JoKeRs_FuRy - PC Specs: Intel Core i5 3.0 GHz CPU, Corsair 750W PSU, Corsair 8GB Memory, EVGA GTX 660 FTW S2 Edition Video Card, inside an Antec 1200 V3 Case