5.7x28 ammo is for sale every day at GunBroker.com. You can buy this ammunition made by FN Herstal through one of the many sellers listing their products here at GunBroker.com, the world's largest gun auction site.
This small caliber centerfire cartridge was designed to maximize velocity. Originally intended to replace the 9×19mm Parabellum cartridge for NATO rifles, the 5.7x28 was not adopted due to disagreements on standardization, even though it was strongly recommended by the panel of NATO experts. However, 5.7x28 ammo is now used by over 40 NATO countries' militaries and law enforcement agencies, including the US Secret Service.
GunBroker.com sellers update their auction listings frequently, so if you're looking to buy 5.7x28 ammo or accessories, check back often.