It's Not What You Know, it's Who You Know
There's a lot to be said for getting into poly after doing as much research as possible. If you're single, you want to figure out what you want so that you can tell each new partner where you stand. If you're already in a relationship, you have talked, discussed, and communicated, and when you were done, you did it some more. Eventually, you're finally ready to dive into poly... so now what?
Just because you've changed your relationship status from "monogamous" to "open" that doesn't actually change anything immediately. You still have to go on dates, find someone you like, see what happens. Where do you start?
Some people start with a friend that they've had in mind since they brought up the topic of open relationships with their significant other. Assuming this didn't start out as cheating that morphed into pressuring the existing partner into "being open" then this can be great. But not everyone is lucky enough to have a friend they're attracted to, who is attracted to them, and who would be comfortable with a poly relationship.
Plenty of people start on OKCupid, I know I did. There are other dating sites out there, and some are even explicitly open/poly oriented. Some people even get started on Fetlife. But for the most part, I've heard a lot about OKCupid in particular. It's open and poly friendly (You can now specify that you're in an open relationship, and your preferences for monogamy and non-monogamy) and it's popular, which means you're more likely to find matches.
Poly Without Pressure
There is an important element of poly that some people miss, however: Poly networking. It's hard to not only find dates and relationships in a vacuum, it can also be very lonely, even if you have a dozen partners. If you meet someone while looking for a partner, there is the pressure to either date or stop hanging out - it can be really hard to meet someone who genuinely wants to be a friend from a dating site. However, if you have a network of poly friends and acquaintances, you have people to talk to and learn from, whether or not you date them (or want to).
It can be easy to forget that you need more than just other partners, but simply people you can talk to about your life and relationships. Sure, you might be able to come out to your monogamous friends, but they won't be able to sympathize in the same way or have much in the way of advice. After all, your friend that has been married fifteen years with two kids might not understand why you're overwhelmed by your schedule being so jam-packed, what with seeing your boyfriend on Monday, then having to head out-of-town to visit your girlfriend on Tuesday, then surprise your fiancée with a birthday party on Thursday.
You also always need to keep learning about relationships in general and how to keep yours healthy. Having poly friends means learning from their experiences, whether good or bad, and being able to ask advice from those with more experience than you. This is especially important when you're first starting out, and can help keep you alert for more toxic partners and relationship dynamics that you might get caught in if you don't know the warning signs. Similar to many BDSM communities (which can overlap) they can also warn you about specific people known to cause issues. They might even be able to gently point out any unhealthy attitudes or assumptions you have before they cause any damage to yourself or your partners.
Let's not forget the other important element of a poly network: Finding dates outside of online dating. Sure, you can try finding someone the typical ways, such as a bar or hobbies, but it can be a lot harder to come out as poly to a stranger that you're interested it. If you're looking for a new partner, your poly friends might be able to set you up with someone they know that you haven't run into yet, and you will have the built-in knowledge on both sides that you're poly or open, eliminating at least that piece of stress. Or, even if there's no one to meet from your group of friends, you'll still have a sympathetic ear about the stresses of poly dating.
Getting There
With all that in mind, you're probably wondering how you find a poly network in the first place. There are a few ways, some easier than others. The important thing to remember is that you should go into this looking for friendship, and putting the idea of new partners on the back burner. Don't get me wrong, you can continue dating, but don't try to combine the two just yet.
In-Person
One of the easiest ways to find poly friends in a low-pressure environment is looking for a local poly group on Meetup.com. It can be tricky to find the exact search term to find a group in your area, but searching for polyamory, poly, or open relationships can get you started.
If you can't find a local group on Meetup, consider starting one. If you're somewhat social and feel like there's a good chance there's other people in your area looking for a group, it might be worth testing out. Creating a group is simple, and start out with a monthly or bi-monthly get-together to get a feel for the area. A nice side effect of this is that you have less of a chance of feeling like an outsider, since you'll all be coming together as a network at about the same time.
Another way to find a local group is through Fetlife (NSFW). While I personally don't like how the site is organized (and the search is always broken!), there are a lot of poly groups on there that don't necessarily have anything to do with BDSM.
This might seem backwards from what I was saying earlier, but networking through partners is also a way to find other poly friends. Maybe it's because your partner has had more time and experience being poly, so they are already a member of an established poly network in the area that they can introduce you to. Or maybe you'll simply meet and connect with your metamours, and their partners, and so on down the rabbit hole.
Online
Finding an in-person poly network can be challenging if you don't live in a heavily populated area, or if you're in a more conservative area. You might also want to make sure that you stay in the closet because of social or family repercussions. This is completely understandable and possibly frustrating. In that case, you might want to stick to online communities.
Reddit has an active polyamory community where you can connect with others and read their stories. In the sidebar you'll also notice an IRC room (live chat) to hang out with people there.
Fetlife deserves another mention here because it's mostly an online community that happens to also be a way to organize local groups. It's more BDSM oriented but if that's your thing, you can also find people to discuss poly with.