I think I know what you mean: A plug (either 12v in-car or 110V/230V mains) that supplies just +5V DC on a USB socket.
Received one with my retractable sync'n'charge cable from time168.com (but those guys dont exist anymore).
Also received two with my BT GPS from DConnex.
I would say they are for sale on eBay, at least with the sellers that supply these sync'n'charge cables.
I made an emergency charger myself based on USB: 4 NiMH AA cells (1.2V each) in a battery-holder & wired to a USB socket. Not pretty, but good enough for emergency.
HTC Vision (Desire Z) with 13th Gen Dual SIM on TMO and Voda
Previously owned MDA Vario IV, MDA Vario and MDA II