It was the guy I was dating at the time who had a bisexual girlfriend he brought over. I think the two of them were in on it and sort of surprised me, but I went with the flow.

So, it was a friend of a friend.

Nowadays, hell, I have no idea where I'd look. Maybe online, but that could be pretty iffy. I'd rather join some social groups where I meet bisexual or interested parties (whatever their gender/sex preference) who might become friends or show some interest.

You could always check out adult dating websites.