Not only are they rarely for sale, when you finally find a pair they are often not matching (small differences between them) I've seen several "pairs" like this for sale, but I don't know why this is so common for scheel seats.
Sometimes they pop up on ebay.de, but I've never seen any of the more desired early 70's 400 series seats (sidebolsters thinner at the top, see pictures below) for sale in recent years, Gerrit helped me find a pair and I'm very thankful for that.
(These are not my seats, but same model)
View attachment 31748
View attachment 31749