Greetings! So I got 13 trophies and LOTS of green and blue mutagens of all sorts (for some reason only red ones are hard to find). I also have the best saddlebags. Nevertheless, I am running out of space. So I was wondering where do I sell thophies and mutagens for more coin? My thanks to you!

Alchemists or herbalists will buy them.

If you look on the Novigrad map & look for Oxenfurt Gate(it's a fast travel location on the east of Novigrad) just North of Oxenfurt Gate you will see a large building, go to this building & you will find an Alchemist who will pay full price for Trophies.
I think the Alchemist's shop is only open between 5am - 6am & he doesn't have that much gold & takes several days for his gold to regenerate.

If you really need Red Mutagens then try & find a Nekker nest & farm some Warrior Nekkers

Level 18+ drowners, Endrega Drones and Alghouls also drop red mutagens with fair frequency. I don't know why they made them so damn rare.

Yup - they're really rare at the beginning when you really need them, and then after you don't need them anymore - they're everywhere!