Grabagun.com has 30 rd Plinker Tactical S&W M&P 15-22 magazines in stock right now for $33.66 ea. Free ship...
http://grabagun.com/plinker-tactical-pt30rd001-22lr-mag-blk-30rd.html
Wow, lots of trouble with the Plinker mags, I hate to see that, I just picked one up...I only use factory mags, but promag and plinker have both been reviewed in the S&W forums.
http://smith-wessonforum.com/smith-wesson-m-p-15-22/
http://smith-wessonforum.com/smith-wesson-m-p-15-22/253500-plinker-tactical-35-round-magazine.html
Lots of info there for 15-22 owners.