i am curious as i cannot find a 5900 series magazine at the local BPS but they stock 411 magazines. They are not even the good MecGars, but beggars cant be choosers. Will the 411 mag hold a 9mm cartridge and fit the mag well properly?

Check CDNN, I have bought numerous 915 mags from them.

Try Gregcotellc.com
Very good prices and very quick delivery.

Original 5900 mags will not interchange with 4000 mags. The 5900 mags were wider and won't go into the 4000 frame. The 4000 mags had a dimple on the front to keep them from going into a 5900 frame. Stick with S&W or MecGar mags there are plenty out there.

thanks for the replies...i figured it wouldnt work...just wanted to try the easy route first. i like to spend my $$ in the local area when possible

just to add to the confusement ProMag makes to fit 5900-915's. they seem to be hit-or-miss, one of them I have I had to 'tweak' the feed lips to conform to the dimensions of the factory mags then smooth with a fine stone to get it doing 100%.
MacGar is good, I have couple of them no problemo.

Good advice. I have several of each and never had a single issue. I cannot say the same about other brands (USA, Promag, Triple K, National, etc.).