Get in touch with Dale Nygaard here on the forum and on AR. He posts under DLN and is presently in Germany, buying drillings and DRs. I am getting one of the drillings he is after and another Kreighoff 22mag insert for it.

There are a lot of used 16ga inserts there, but 20ga inserts are uncommon. They are made and work well. I had a full length in 22 hornet that was amazing.