You may have to by a whole assembly. I found one on e-bay last year. I made a similar mistake in not backing out the set-screw first, before trying to back out the plunger and snapped the 2mm hexhead in the back. Luckily I had enough room and with a rubber band for added friction, was able to remove the plunger and transfer it to my new lever. I got the clutch lever w/ plunger as insurance in case that original part failed - so far it's holding up. I checked places like Beemer Boneyard to no success. E-bay was where I found the lever needed. I think it has to do with BMW Motorrad's liability and parts control - that you cannot get that plunger bit on its own. (My speculation). It was about $100 (yes, I paid it) - chalked it up to a painful lesson learned on my end. There was a very similar post to this on Facebook on the MOA page.