The aftermarket bottom metals made by Atlasworxs or Mountain Tactical will fit either a standard T3 stock or CTR and both accept AI pattern mags. Get the stock, check which bottoms metal/trigger guard it has and then get the corresponding aftermarket shape that takes AI mags. Just remember there may be a bit of fettling required to get the magazines to fit and feed perfectly.
Personally I prefer the shape, mag release and ergonomics of the Mountain Tactical and they're a good company to deal with.