Like many a new gamer before me, I've made my way to Etsy to find some pre-made pieces that I can use for our upcoming game! When it comes to holsters, there are lots of options if you want to go with leather, but not so many for a more modern/tactical set up. However, I did find one store that makes such a rig, and for cheaper than leather (and much snazzier than the Blastersmith UK ones).
He had ones pre-made for a Maverick, and even the damn Retaliator rifle. So, if you want something tactical without having to tear apart some airsoft/actual gun gear:
https://www.etsy.com/people/apexweaponsystems
I got a drop-leg holster for my Sweet Revenge/Hammershot (Marcus, the shop owner made it after a couple of questions from me), for $25 bucks. That's a better price than even the Army/Navy stores 'round Boston have.