M1.8? Interesting size. But I see that M1.6, M1.7, and M1.8 are all out there. North West Short Line has M1.7 set-screws:

http://www.nwsl.com/Catalog/pg105-cat8-01-v0806.pdf

Micro-Tools may have one as well, but their online catalog is useless, as it doesn't give the sizes of any of the screws!