CDNN has all three sizes in .40 and 6 round 9mm available.
I'd also check 44mag.com & Gunmagwarehouse.com. Brownells isn't cheap but seems to be as good as Midway for keeping stuff in stock.
But the last few pistol mags I've bought (Ruger LC9S & M&P Shield) have come from ebay, there are usually more than a few sellers with the lowest prices plus free USPS shipping.
Just for comparison, I looked on Cheaper Than Dirt's website, ordered an 8 round Shield mag, a 10 round Promag and some Pearce grip extensions. About $60 worth of stuff, but the S&H was $18. No, no, no.
Ordered the same stuff from ebay, total was about three dollars more but free S&H on everything, so saved $15.
So....are the mags for the new PPS M2 different from the M1s? If so, now might be a good time to grab a few more M1 mags...
[EDIT} Nevermind, I just saw a pick of the M2 mags, they don't look like my M1s...