Yep it never ends . Welcome to the club
That PSA deal is not bad . You should be able to find mags all over the net at good prices including the Magpul ones with the windows . If they are for the AR15 , 223/5.56 they should work in your M&P just fine . Gun shows are a good place to pick up mags as well. I get alot of my bulk ammo from this site ( free shipping on all bulk ammo ) http://www.targetsportsusa.com/
. They also have other stuff like mags . They are not always the best price but they seem to stay competitive .
Unfortunately Im in C.A. and can't have mags that except more then 10 rounds . This makes it much harder for me to get mags at a good price . If your going to do any bench shooting useing a rest . I also would recommend you getting a few 20 or even 10 round mags . The 30 round mags can stick down to far and hit the table making it hard to get comfortable on the bench/rest .