Some people find partners on OK Cupid, a dating site. Others go to poly meetups/munches. Sometimes you just might meet someone randomly who is open to the idea...
It's best to tell a potential partner that you are poly asap.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags, F, 60, poly-dating, loving and living with
miss pixi, F, 38
Punk, 41, M (dating since Oct 2015)
and a few more casual relationships