I think you are going to have to make your own. The only ones I have ever seen are the Fidelity Research ones. I think they were FR - it was a long time ago and I haven't seen any since.
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The K-5 cases are available on EBay and HiFiDo (under Accessories but be quick to pounce) and you can sometimes find the 10 cartridge holders which were made by the company that originally made the K-5 for Fidelity Research.
The internal foam is very rugged if it hasn’t been abused.....