4506 magazines work just fine in the 645 and are actually more reliable and more durable that the original 645 magazines. Obtaining magazines these days boils down to what you can get or cobble together from parts.
My advice would be to stay away from magazines that have a metal floor plate that is retained by a small tab spring at the bottom edge of the magazine body. These floor plates have been known to slide off during a hard magazine change. Metal or plastic base plates that have the retention button in the bottom of the plate are fine.
The original metal followers can also cause issues with feeding, but the plastic followers are much better. The first of the plastic followers was orange, then came yellow which were to be used with magazines stamped with a warning to use only yellow followers. The last version, and I think these work in the yellow follower magazines too, is the black follower.
Get ready to spend some money, genuine S&W 645/4506 magazines are not cheap these days and I do not recommend the ProMag magazine... unless you feel lucky or like to practice clearing jams.
