As Horace Greeley would have advised,
"Go Google, young man."
Internet search is truly your friend.
Not once, or twice, but often.
Search until you may search no more, then...
search again.
That's about what it takes. I think it took me a week of constant checking with all sorts of sites before I found the 4 extra mags I wanted for my Shield. Don't think because you checked one that you should give up, either...they might put some on right after you check. Good luck!
Tip: If you haven't looked there, there's a place called "The Shooter's Box" near Boston that occasionally carries those older S&W magazines. They have a web site.
Edit: Yep, they do, for $40... sorry.
Could be worse... Taurus semi owners have to wait months for the boat to show up from Brazil with their mags, with no guarantee there will be any.