Hi there - thanks for asking.
If you want a relationship that is really just about the sex, then you need to be looking at swinger forums online - most of the polyamory forums will be talking primarily about loving, romantic relationships (which of course can include sex - sometimes lots of it!).
So if it's someone to have regular threesomes with without anything else, really, other than a friendship, your ways of finding them are a lot different from a full-blown romantic relationship between you.
The other thing to think about is what happens if you meet a girl and it totally "clicks" with her and your wife, but less so with you and her? (Or the other way around).
Lots of things to get sorted out in terms of what you do and don't want - the more you and your partner can work on before you meet another, the less drama and disappointment you are going to face, and the less time you are going to waste...
"Listen, or your tongue will make you deaf." - Native American Proverb