Not sure I've seen any Kahr after-market magazines before, but I always check CDNNInvestments.com first when looking for more magazines, they usually have the best price and often have different deals, like free shipping or a free ten rounder with purchase of a regular capacity magazine, really appreciated that for my USP. $40 magazines don't hurt so bad when you get a ten rounder with it.
Also they often throw in a cheap knife or flashlight with orders, or glasses, so even though that isn't really that valuable, it just feels nice to get a present. For manufacturers, the only after-market maker I trust 100% is Mec-Gar, they rock. They are also the factory magazine manufacturer for many brands.