To supplement my income I provide a taxi service for the senior citizens in my building, taking them shopping, to their doctor, etc. Some of the clients formerly lived in the questionable areas of town and still have contacts or obligations there. So sometimes I spend an hour or so sitting alone in my car in the gang-infested parts of the city, waiting for a client to finish his/her business. All this by way of explaining my search for a shoulder holster.
(I'll also mention that concealment is paramount: having a gun in my apartment building is grounds for eviction - and I hate moving. So I can't even let my passengers know I'm carrying.)
Though I can pocket carry (always in a suitable holster) my Ruger LCP or my Taurus Slim, I've learned that getting them out of my pocket while sitting in a car involves several contortions - movements I probably won't have time to make if suddenly confronted by a thug or a car-jacker.
For the same reasons, OWB and IWB are not solutions I can use.
So I've spent a lot of time searching for something that works. One case in point will illustrate the difficulty: Recently I purchased a DeSantis Patriot shoulder holster. This is nylon with nylon straps and while DeSantis makes many fine products donning this one felt like I was trying to insert myself into a spider web: so many adjustable straps and buckles and by the time I adjusted the straps to fit I had long ribbons of excess material going off in several directions. (Of course, I could have trimmed these once I had a proper fit.)
Couple that with the fact that properly adjusting most shoulder holsters requires two people and you get the idea.
When I was a juvenile delinquent I carried (for a while) a Marksman air pistol in a shoulder holster. All I remembered of it was that it was good leather and must have been easy to get on and off.
Finally found it: It's a U.S. Army M7 (really a chest holster) with a single strap that goes over your head and rests diagonally over your chest. Though made for larger guns (1911s) a holster not already stretched worked fine for my Slim.
The newer M9 holster also works. This has a single strap that goes on your left shoulder and a band that encircles your chest to keep everything in place.
Either can be concealed under a jacket, so long as you keep it zipped up enough to hide the strap.
More recently, I learned about the Kangaroo Carry holster. Very simple affair made of cotton cloth and velcro. I now have two of them: one will fit my Slim and my even larger Cougars; the other has a smaller pocket and is made for my LCP.
I won't try to describe them except to say they are comfortable, inexpensive and do the job. Pictures and more complete descriptions are on the wensite:http://www.kangaroocarry.com/
As for the M7 and M9 holsters, replicas are made by several companies but I found the best buys on Ebay. Just search on "shoulder holsters" and you should find plenty at varying prices.