Bob, I can understand that you'd like to swing with another woman. Are you dating? If so, then you know the trepidation that comes with asking a woman, "Are you interested in sex with others." Since we in ENM are just a fraction of the population, just bumping into such a woman is way against the odds.
And being a single man is no picnic. You've got a leg up, I think, in that you'd be empathetic to what the other woman is thinking. But you'd still got to find such a woman (or a couple,) and get through the competition.
Would it be possible for you to look up the partners of your past, explain that your wife is no longer with you, and ask if they have any contacts that might want to meet you? You might also wind up getting invited into threesomes, or house parties or such where you can meet eligible women.
Good luck.