I would suggest searching for people rather than structures. You never know what wonderful things are going to come into your life if you're open to them, and conversely you may hurt a person or a relationship by trying to make it into something it's not if you have lots of preconceived notions.
Maybe try setting some boundaries for yourself to start. Examples could include:
- In order to protect my health, I will only be involved with people who are willing to tell me about their other partners, tell their other partners about me, and let us meet each other -- I will not participate in any "don't ask don't tell" or cheating-based arrangements.
You can build on this as you go along, but simple things like that give you a place to start.
Next, start setting out to meet potential partners. You could join a poly-friendly dating site like OkCupid, or you could join a poly discussion group, or you could just start getting involved with new activities while being open about your intended lifestyle. Just get into contact with people with an open mind and heart. Where friendships develop, wonderful! These are great things to have in their own right, and can also help introduce you to new potential partners. Where it feels like it could be more than friendship -- mutual attraction, alignment of values, affinity in personality -- be straightforward with the other person/people and see what happens. Take any "no"s with grace and keep looking.
You'll find the right structure for you, whether it happens right away or after a few false starts, and whether it's by getting involved with people who are part of existing poly networks or by starting your own. Again, start with people, and let the best structure reveal itself based on the unique blending of your personalities and life circumstances.
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.