To these one might add the Kelleher auction house.
https://www.kelleherauctions.com/ Over the last year, they sold quite a bit of error material from Bill Langs, and one might reasonably suspect they will auction still more that keeps trickling out of the vaults.
BTW Langs stock sold via Hipstamp is actually fulfilled (pulled, packed, and shipped) by Kellehers, as I learned when I bought some blocks via Hip of 900a and 901a in order to even out a custom album page I had going on the perf errors of the Defense Issue.
- Jonathan