I never came across them. Now i saw a mission for them but I do not know in which kind of places I have to look for them Asteroids maybe? I never mined in asteroids...
Rutile and cobalt can both be bought in commodities markets extraction and refinery respectively.
You can mine Cobalt from Metal Rich and Metallic rings and Rutile from Rocky rings around planets.
I think (not 100% sure) that they are mining-only, which means they can't be bought on the market and you have to mine them for yourself, just like Platinum or Painite, for example.