As ripper said HID could not be standalone bulbs since they require separate system to operate the Xenon bulb. However I have seen most of ppl selling some bulbs saying its same as HID. Be aware when trying to find "standalone bulbs" giving more light since most of them are completely different from HID concept and consume more power than a normal bulb causing damages to plastic head lights, reflectors and the electrical system.
Before looking for HID, just find some info about color temperature and how it affects when driving. HID are having higher color temperatures and when it reaching more and more high, it becomes useless.