I picked up this DeSantis IWB holster from Midway:
Intruder Inside the Waistband Holster Right Hand S&W M&P 9mm Luger 40 S&W Kydex Leather Black
I have the M&P 45c and wanted to give it a try, even though it was made for the 9mm or 40S&W. The 45c fits perfectly in it in my opinion.
From what I was told in talking to my local gun shop, there are 3 models that have the same exact main frame [45 fullsize (4.5" barrel, full grip), 45 midsize (4" barrel, full grip) and 45c (4" barrel and compact grip)]. They all share the same frame and there are only minimal differences in the slide length and grip. If you look at the pictures of the different models, the slide hangs about half an inch over the end of the frame on the full size 45. On both the 45 midsize and the 45c the slide is about even with the end of the frame.
Based on this, which could mostly be assumptions, if the holster does not cover the muzzle and does not enclose the grip, then any holster made for any of the M&P 45s should be interchangeable.
So, not sure if this helped at all or makes sense, but it kind of made sense in my head...
Good luck.