Ebay I have managed to score some deals on 4506, 3913, and 3906 mags if you look through them all. I usually spend ave 30 bucks with ship on the ones I win.
JMO, but always keep an eye out and (presuming funds are available) grab 'em up when ya run across them. I have a couple of old autos for which I maintain a 'vigilance' for extra mags. Over the years, I've managed to collect enough extra new parts to completely build a new model for each. For me, it's easier than hoping I find the right part at the 'last moment' if something ever breaks.
Just an update, the new magazines came today, about 5 days sooner than expected, that's great.
They are brand new, unused magazines. Kind of pricey, but what the heck, I gotum.
You would think that the manufacture would check the fit of the spring to the follower before or during assembly. One was way off and the other wasn't much better, but it was easily fixed.