Canuckians were buying tac shotguns from Brownells back in 2013? Anyway, if a US dealer is wanted, Cabelasbasspro has M4s in stock, with a 10% discount if you have one shipped to a store for pickup. Unless they've cancelled that last bit, check online.
Re M4 stocks, I've had both types. Both have their advantages but having been asked, I have a slight preference for the fixed tacical stock with pistol grip, not really expecting to be handling the gun in tight spaces like say inside the house. However, if that did happen, the pistol grip would allow you to provide enough resistance to the recoil to allow the gun to cycle, which it does at all times anyway.