Welcome! The partners and poly lovers I've had over the last several years have been friends, friends of friends, and one lovely gentleman met at a BDSM party. So, I'll second NR in saying that making friends is probably the best way to go, with no particular expectations that the friendship has to be more than that.
There is probably a poly social group in or near your area if you do some internet searching. Some people here have also had swinging-friendships turn into more, though I'm not sure how common that is? I guess one big question is, are you just excited about the idea of finding a lover or lovers that can also be good friends, or are you open to one or both of you falling in love and forging a real relationship or relationships with new people? There's nothing wrong with the former, but if that's the case the swinging community may offer you more of what you want than the poly community, and it's good to be aware that love, not entirely unlike an std (forgive the analogy), can never be 100% guarded against.
If the latter, it's great that you guys are open to what you find. Experienced poly folks tend to shy away when they hear "we want a girl to join us" or even "we want a couple to join us" because it implies a mindset of "couple plus 1" or "couple plus 2" that ends up tripping people up when they see one member of the couple falling faster than the other. Love is unpredictable, and while threesome or foursomes aren't too hard to find, trying to develop loving relationships between all involved can be a huge mess. Staying truly open to what you find, and not having any expectations that things have to happen with both of you present and involved, will be the best way to go. I find this set of advice, from the point of view of the person outside the couple, to be particularly useful: http://www.morethantwo.com/coupledating.html