If you're either young (mid 20s or under) or in a high cost city where roommates are relatively common even for single older people, you could post on social media to try to find a friend of a friend looking for a roommate.
From your other post it looks like you could be considering buying a house and renting a room. Before making such a big decision, remember to consider rules you'll have to make for each other about having friends and especially dates over to the place. Unless you can divide it up that they essentially have a basement apartment or something. That's why a stranger roommate isn't worth it to me in my low cost area. (I would probably room with an existing friend if I knew someone who wanted to though).