Yes, Go to Earl's Repair Shop - the thing is - they are $10 each, and he has a $25 minimum order.
http://www.carlwalther.com/p99as.htm (look at the last item forsale on that page)
I prev contacted S&W/Walther USA directly, and they could not help me. Others here have stated that they were mailed some from S&W, but when I contacted them, they claim they get the mags in 1 complete unit from Europe. SO, they said they couldn't do that, and denied that anyone else had ever received any of the baseplates from them.
Luckily, someone on AR15 was nice enough to send me 1 baseplate for free. So, I have the finger extension on both of my mags now. 2 mags are enough for me, since I also have 5 full size ones if I wanna use them at the range (I have a full size SW99 too)