Welcome to the Board, Ymie1.
What you are looking for has been referred to as the "Holy Grail". Virtually almost impossible to find. Now this is not to say that it can't happen, but it will not be easy.
Although a couple may seem a little intimidating there are a lot of them out there looking for the same thing you are and that will probably be your best bet in finding one who would allow a FMF while the husband of the other woman watched and then changing and allowing the other couple to also have that fantasy happen for them as well.
This particular topic has been discussed many times here on the board so look through some of the other threads, I'm sure there are other suggestions on finding a single female in them.