If looking for a bulk buy, ULine has them at 18 cents (US) each, but with a minimum lot of 50 units (25 pairs). Available on-line in red, black, blue, green, white, or clear.
I also found them in person at my local gigantic home improvement store, Home Depot, as well as a smaller local hardware store being sold as "thread protectors" for about 75 cents US each. These were gray.
REF: ULine 1/2" end caps
edit - Really good looking R500!
Edited by - BBL on 7 Jul 2014 01:49:47