wife wants a 2nd woman in our triangle... how do you even start

wife brought it up a few months back, she wants the experience of a 3rd, specifically another woman, with both of us involved. i'm all for it but i have literally no idea how to even start looking. apps feel weird, we dont really want a unicorn hunter vibe, we'd rather it grow naturally from someone we know or meet at an event. clubs we've been to seem mostly couple-couple. has anyone actually had a triangle with a 3rd female form organically, or does it always have to be sought out... we're in the north west uk if that helps