@namechsnge - I don't necessarily agree with @FortunesFave.
I am an occasional third for a happily married couple. When we first met, they were quite clear that there would be no PIV - if things progressed it would be "soft play" (think lots of foreplay, including oral, but I wasn't to cum in her mouth)
I agreed to this, as I wanted to be involved in a threesome. Our first evening together was one of the most interesting and sexual experiences of my life, and it proved to me that sex is not all about PIV.
Since then, the dynamic has changed, and PIV is very much on the cards, but they both had to make sure I was right for them, which the soft play session allowed.
I would say, decide on your "must have"s and "absolutely not"'s and stick to them.
As to where to find someone, the couple approached me on Fabswingers. They liked my pictures & profile, and messaged me asking if I would be interested (Yes please! 
)
We met for a social (very important!!!), and we all decided we wanted to take things to the next level.
There are thousands of single guys on Fabswingers. If you decide to go that route, set your filters carefully, or hide your profile from everyone, look around, and approach people that you find interesting.
You could try Feeld, or OK Cupid, but these are your typical swipe left, swipe right dating sites.
KillingKittens is an option, again lots of single guys, but it's female led, so the female has to make the connections.
Feel free to pm me if you think I can be of any more help in your search, and no, I wasn't offering to be your third! (I'm probably far too old, too ugly or too far away)