As already mentioned, Mec-Gar or genuine S&W 59/X59/59XX magazines only. Do not fall into the cheap, Pro-Mag trap as they are usually junk.

The very first thing you want to do after a good cleaning and proper lubrication is to install a new recoil spring. The factory rating is 14 pounds and that is all you need to use. Genuine S&W springs are inexpensive and easy to find, if you want a higher quality spring, get a spring made by Wolff.

Make sure your Model 59 is reliable with your self-defense ammo. Early production Model 59's sometimes had reliability issues with hollow point ammo, but yours is probably late enough in the production run to be reliable, but you should still test to be sure.