Thank you guys for all your help! I know how to use google. Those holsters, that are suggested by Scrod's google search, can not be attached to the backpack strap. In fact those are not holsters. Those are just sealed pvc bags.
What I was looking for is something like this: http://www.chromebagsstore.com/bags/...ory-pouch.html
Unfortunately this chrome's holster is the only thing I was able to find, and I was hoping maybe someone here knows the alternatives.