Edmund Scientific carries it. I haven't looked in a while, but seems like they were expensive - but then again I haven't priced it anywhere else either.
The have 2 branches (or 2 faces anyway). One is optics-only aimed at the industrial laser crowd mostly, and the other is general scientific gear, more educational. Both sites/catalogs would probably have it, but the optics side is probably going to carry the highest quality and price.
PS - let us know where you find it, I plan to rework some Kalart rangefinders