I don't have an IWB holster for my AAC - but I do have one for my 4.25" and 5" carry pieces.
I got mine from Brigade Gunleather and got the Custom M-11.
The holster took about 5-6 months to get hand crafted, but when it showed up it fit like a hand going into a nice, comfy glove. It was a PERFECT fit and it wears really, REALLY well with my Milt Sparks thick carry belt. I love it and it's my only carry holster now (out of about a dozen I have tried).
Now with the rail and the extra tall sights (which you can dump if you go with the Osprey ) you will probably have to send them a pattern. I know they do custom work, so it might take a little longer.
Good luck. Buying a holster that you like is one of the most personalized decisions so I hope you end up finding the right one before you spend too much cash.