Removing is easy, a bit of heat and toothfloss makes it a piece of cake, then it's a case of peeling off the excess, again some heat from a hair dryer or better yet a heat gun will really help. The OEM M badges are a damnsight sturdier then the aftermarket ones, the rear one is also metal.
I went for ones from BMW and for a full set of roundels and M badges it cost be nearly £250, no doubt aftermarket ones are cheaper on Ebay, depends on the result you want.