Try following this guide.
The first part of the holster is at the gas station but after that you need to take control of different control points held by the different fractions. Since you keep getting the same part might try that and see if it drops after that.
I have the exotic holster but not sure where/when I got all the pieces. After the warlords update I was able to buy the blueprint and craft it. Since then it has dropped a could times just doing missions in the game.
Hope this helps.