Have a look at some fiber shops. Which country are you asking for?
The Limit is, at very first, the SPF Module. Secondly, at such long distances, the cable quality is important. Take
good quality(!!!) Single-Mode Duplex Cable, select the the ones for long wavelength (~1300nm as i remember).
Most of the SPF Modules should also work for Mikrotik boards. 80-100km is not a Problem for the the good Modules (more expencive but less fault tolerant). They are most likely sold as Dell or Cisco and running a Broadcom Chipset. No guarantee to work, maybe you can ask the shop to test them.
You are not asking for a router matter, you are looking for a matching SPF Transciever Module for your Mikrotik Device
Best Regards,
redflag