I have 2 M-3's. One just stopped working. I switched the heads and batteries, so something is wrong inside. I contacted Streamlight. They said they don't work on them anymore and would have to get ahold of Insight Technology to see if they will work on it. In the meantime I bought one of Insights new LED upgrade for the M-3/M-6 weapon light. I'm very satisfied with the upgrade. The old bulb you can find but I wouldn't bother. I think the upgrade brings lumen count to 125+. it's comparable to the TLR-1. The only downside is the price. They run about $51. I got mine at GT distributors during a storewide sale. I think they may have been a limited production run being they are pretty hard to find and came out about a year ago.
Try to find a pest control distributor. They usually sell flashlights for rodent investigation. univar, residex or John deer/lesco. I used to work for one of these here and we sold some parts for streamlight and surefire lights and some of the bulbs for the stylus type lights work in the tactical lights.