A friend just sold a set for 1k back in November but I wasn’t looking then.
You have to use the old traveler's mantra. Eat before you are hungry, and take the opportunity of a rest stop even if not necessary.
If you are doing any kind of rebuilding or restoration, think like a chess player for all the moves ahead, and strike when necessary.
To ease your burden, a 5.5" is fine for the spare. That's what I have. About the 6" ones, I initially found a set of two, then a third, and had the pair powder coated, and eventually sold them to our past president Ray Hays who I think had a line on two more. Then I still had that one single 6", when a friend offered me a full set of perfect 6" ones off of a TD that he had bought for a church (the church didn't care what the heck kind of wheels it had). I paid his junkyard price for a set of steel wheels to replace the aluminum ones. At the first PUB I went to with the new set of freshly powder coated 6" ones, I brought along the solo wheel too. Founding member Tom Sargent had 3-6" wheels, and one 5.5". I simply swapped him the 6" for the 5.5". The 6" one completed his set, the 5.5" became my spare. Everyone happy. A few bucks changed hands for some refinishing and the wheel cover clips, but no usurious pricing. Just good friendly camaraderie and furthering the cause.
The essence of club membership.