Does anyone know where I can get these knobs?

They go to some butterfly dampers on some older circular ceiling diffusers. I am missing a few and having a p.i.t.a. time finding some replacement ones. I found a thread on another forum where someone was looking for the same exact ones I am needing, but the was no mention of finding them.

Anyway, I borrowed the photo from that thread as its pretty good. I know I'm not the only one needing these!

Thanks