Tons of these rifles have been imported to the USA in the past 15 years but spare magazines are almost impossible to find. There is a K11 magazine on gunbroker right now with a starting bid of $185 US$.
Switzerland is probably your best bet to find one at a reasonable price as the stuff is cheapest of all there.
There is Edelweiss Arms here in the USA that has built up a tidy business in Swiss arms, I see they have K11 magazines, you may want to contact them and see if they are willing to ship one.
https://edelweissarms.com/accessories-parts/magazines/
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