Other vendors will make magazines if demand exists. MecGar already makes mags for the 59 and its descendants, including 17rd mags that work great.

Check ProMags offerings. They probably already offer mags for the third gen .45s. Their quality, at least on metal pistol mags, has become better in recent years.

Parts for Colt D frame revolvers can be had from Armscor. They bought the rights to make the things and still turn them out in the PI, albeit that even they admit that the end product isn't quite as good.