I'm left handed and carried a Glock 17 in a Safariland Top Gun Holster. I have a level I and later went to a level III. I've never had that problem.
If you look at both the left hand and the right hand holsters, you may find that the magazine release is exposed on both. The problem is that the left handed officers sometimes bump against something and it hits the magazine release. It's easier to do because the magazine release is now on the outside on left handed officers holsters and less protected. My biggest problem was catching the seat belt on the holster and pistol when I was getting out of the car. I had to modify my actions by moving the seat belt completely out of the way before getting out.
There are several manufacturers out there that make holsters. Safariland, Don Hume, Bianchi are the top three that come to mind.
You may also find what you're looking for in the Galls Catalog:
www.galls.com
Also, check www.officer.com that site has bunches of links to Police Equipment supply.