Hang out in different places. Go to writer’s nights or other events where potential co-writers will be. You never know. I had one cut with the B.O.B because he saw me perform at a songwriter night and one cut with Snoop Dogg because I got the chance to meet him at his backstage in Malaysia and rapped for him.
Come in to every co-write prepared. Word gets around. If you come loaded with ideas and ready to work, you’re a valued co-writer.
Give lots of people a chance. The more co-writes you have, the more you learn about the KIND of co-writer that is best for you.
Be honest with yourself about what you do best. The more accurate you are in regard to that self-assessment, the better success you will have in finding co-writers who fill in the blanks for you. I’m not great with melodies or guitar, so I write with people who do those things really well. That allows me to focus on what I do best.