Ah, well, there is no such thing as the perfect woman. And it will be especially hard if you want that perfect woman to "add to" your existing relationship - like a condiment added to a steak!
There are a gazillion threads that could be helpful to you, if you do a tag search for "unicorn," "third," "triad," etc., but here are a few to start you off:
Added to, Joining In
Why Is it So Hard?
Frustrated M/F Couple
advice for couple seeking third