XPR equates to a Xenon gas PR base bulb. 18Volts at .6Amps...Kind of a weird one, because of the voltage. I would check Sears, or look for a similar bulb from a Mikita light, i.e. from Home Depot.
As a last resort, you could use a PR-18V, which is the same, but w/out the Xenon (you'll lose some brightness, though)...
--dan
P.S> I'm moving this to Modifed, for more results..