I have a Mike's IWB holster for the G20/21 that works well for the M&P40. My gun fits the holster very well, but I don't like the holster much at all. It was $13, and I bought it just so that I could take my time finding the right holster for the gun.
After I find the perfect carry holster for the gun, I was thinking about drilling a hole into the Mike's holster and mounting it on my bed frame, or at an accessible place inside my car.