At least if a wedgehead back door is missing, PBR can sell you a repro. Mr. Young doesn't carry the multiplayers...I got very lucky in Allentown this year and found an original two-player door for my Solar City among the twenty or thirty of varying sizes leaning against the far wall of the indoor swap area. There is an ebay seller, I think it is the "Jeff" mentioned above, that has single, 2- and 4-player doors available once and a while, but communication is hit or miss and he hasn't listed them in quite some time.

I'm also on board with the frustration of missing lockdowns...they were missing on the last three project machines that came in the door...