I believe that CDNN still has some of the 12 round 40's in stock at a good price..
http://www.cdnninvestments.com/

As to quality of mags, there are only two types:
1). Mecgar, who make the Walther and S&W branded mags - excellent quality.
2). Every other brand - Junk.

As I stated in another post here.
You can buy the SW P99 or SW99 magazines for your pistol. They are the same thing, and will fit in your P99 as exact replacements, they will just say SW on the bottom rather than Walther.
I have looked at the MecGar sight and they do not make magazines for the Walther P99.
Some of the other Walther guns, yes, but not the P99.

ProMag does make 15 round magazines for the P99 9mm.. And while I do not normally recommend ProMag, I have some of these for my P99 in 9mm and they have worked flawlessly for me.
http://www.promagindustries.com/Prod...uctCode=WAL-A5
The link above is to the ProMag sight for 40SW mags. You can try them, they may work as well for you as my 9mm mags work for me.
Or again you can buy the SW99 magazines.
I found some at this sight
http://www.wholesalehunter.com/produ...roductid=52447
and here, 4th item down
http://www.shopwiki.com/TRIJICON+N%2...9+(B000LJYNSK)
Prices are hi for the Walther and Smith mags, so you might want to give the ProMags a chance, at least for range mags.
Good luck.