IIRC the only mags that will work are 3906 mags. I had one long ago, and it's one of the few guns I'd rebuy today if I ran across one. I'm pretty sure the 3906 is a hair longer in the grip than the previous gen pistols, and early gen mags end up not locking up. The 3906 mag, being longer will work in the earlier guns, but the earlier mags might not work in the 3906. I might be all messed up on this, as it was long, long ago, but I'd make sure with some test fitting before you order any through the mail.
CDNN is usually the first place I look for mags. Midway carries them at $30, but they're on back order.
I think $20 for a S&W factory mag is a decent price.
A pawn shop nearby had some 3906 mags for $20. I'll check today if they have any left. How many were you looking for?