Here are some pictures of how some commonly available high-cap mags fit in the Walther P99 frame. As you might guess, some fit better than others.
Beretta M9 / 92f
The Beretta mags fit fairly well, but there are a few problems. There is a fair sized gap between the frame and the floorplate, and the stock follower does not engage the slide lock very well.
CZ75
The CZ75 mags seem to be the best overall fit so far. The gap is smaller than the others, but the follower engages the slide lock way, way too early. It will require, at the very least, a proper follower.
Sig 228
While the Sig 228 mag is a decent match for the size of the P99 mag, the floorplate keeps it from seating far enough into the frame. A modified 226 baseplate may help with that, but I had no donors to bend up.
Sig 226
The gap between frame and baseplate is a little bigger than some, but the fit is not too bad.
S&W 59 series hi-caps
I had an opportunity to test-fit a Smith & Wesson double-stack stainless mag at the show last weekend. I'm fairly certain it was for a 59 series. The fit was almost perfect length-wise, but the top of the mag was too narrow. It wobbled back and forth in the top of the magwell. I do not think that this mag, even with a proper follower, wil engage the slide-lock properly.
Ruger P85 / P89 / P95 series highcaps
Last weekend I made it to a show... At the close of the show I found a Ruger mag that seemed to fit fairly well in the Walther frame. The follower did not engage quite right, but replacing the followers with real Walther/S&W parts is an expected requirement.
I'm not quite certain which Ruger mag it was though... They guy thinks it was a P89, but we had no way to be certain. I'll be looking for one of provable pedigree next time I'm out. When I have one up-close and personal I'll see if it's a better fit than the CZ-75 mag.
If you know of another magazine type that I haven't mentioned that might fit or have pictures/info to share on mags that do fit, drop me a line at Cranky@CrankyLabs.com