I've had good luck with the Dorcy LED replacement bulbs. It's the standard size bulb found in just about every old flashlight. There's 3V and 4.5-6V models. Great runtimes and nice white light.

I don't think I ever paid more than $4 for them. Sears and other big box retailers used to carry them. Should be some good online deals, provided they fit your light.

http://www.dorcydirect.com/p-353-41-1643-30-lumen-3v-replacement-bulb.aspx

http://www.dorcydirect.com/p-354-41-1644-40-lumen-45v-6v-led-replacement-bulb.aspx