$10 at O'Reilly Auto Parts. If I recall correctly the diameter is .24. It easily fits the bore of my .32 if I remove the pocket clip. It comes with the batteries. Replacement batteries are around $6. I use mine for gun inspection (never go to a gun show or pawn shop without one) and to walk to and from my deer stands in the dark. The original batteries lasted a little over a year. You can turn the pocket clip around and clip it to the brim of your hat.