You find poly people to date in pretty much the same way you find anyone to date: by putting yourself out there, being your best self, and being patient!
Meetup.com lets you search for polyamory-themed meetups, as well as meetups in your area that fit your other interests where you can meet potential partners!
The Polyamory.com forums have local sections for personal ads.
There are some tumblrs that run polyamorous personal ads! Polyamblr is mostly user-submitted personals, and Poly Play Date has a live chat and various user-submitted posts. Poly People Place is a personals blog that just got started.
Poly-specific dating sites:
I have no experience using any of these sites, so be aware they might not have certain functionality or a large userbase.
Finding local groups:
Often, especially in cities and progressive areas, places like sex shops and dungeon spaces often host classes and workshops on non-monogamy and other related topics.
Try Googling for event calendars and sex-positive venues in your area and check out events that interest you!
Poly Advice posts on this: