Since getting spare Ten II magazines is such a pain, I decided to find out if the more common (and cheaper!!) Para-Ordinace double-stack 45 mags will work.
The answer is Yes an No. Yes, they'll work, but not without a little effort.
After comparing Mec-Gar P-14 mags to the factory Kimber mag, I bought a couple. It was obvious they were manufactured on the same production line, and the slot for the mag catch is slightly lower than the factory Kimber's location. The p-14 mags fit just fine, but would not quite lock into place. Measuring the difference I found I needed to remove ~0.055" from the upper side of the P-14 mag catch slot. They now work just fine - they feed & function as well as the factory mag.
One other note - The factory Kimber magazine would not hold the slide back after the last shot. The little step on the magazine follower is lower than on the P-14 mags, and it does not even touch the slide-lock. I swapped the followers between a P-14 mag and the factory Kimber mag, and the Kimber mag now works well with the P-14 follower. I called Kimber support, and they mailed me a replacement follower, free of charge.