10 mm ammo is for sale every day at GunBroker.com. You can buy this ammunition made by manufacturers like Winchester and Colt through one of the many sellers listing their products here at GunBroker.com, the world's largest gun auction site.
A rimless cartridge for semi-automatic pistols, the 10mm Auto is a very powerful cartridge with the flat trajectory and high energy of a magnum revolver cartridge, yet is still relatively small. Originally designed for use by the FBI, it was later decided that 10 mm ammo had too much recoil and only compatible with larger pistols than the FBI preferred to make standard issue. There is still, however, a loyal fan base for 10mm ammo among civilians and some law enforcement agencies.
GunBroker.com sellers update their auction listings frequently, so if you're looking to buy 10 mm ammo or accessories, check back often.