An invisible displacer beast that you can’t see should not be able to use its displacement abilities in combat with you. (If you have see invisible, it should still be able to use them.)

Displacer beasts could drop their hide, which can be enchanted or otherwise crafted into a cloak of displacement.

Displacer beasts may drop a hide, which can be enchanted or otherwise crafted into a cloak of displacement. (Or else this could just work with a displacer beast corpse which could be enchanted/polymorphed/crafted, without requiring a new object for the hide).