Best way to find a neurodiverse cofounder?

I have ADHD does that count?

But more seriously, I don't think there's a substitute for talking to lots of people. I'd probably avoid looking for a "neurodiverse" candidate so much as looking for a person who fits extremely well to the problem and your missing pieces. Maybe that's someone who's "neurodiverse", maybe that's not, but IMO there's no substitute for talking to a lot of people in the space you're trying to enter.

Do interviews that aren't boring questions everyone is prepared for, and you'll be able to tell if you get them to actually talk to you instead of following the rules of what is professional to say in an interview.

I have ADHD and am probably on the autism spectrum. Does that count?

Honestly though you're asking the wrong question. Why do you specifically want a neurodiverse cofounder? What problems can they solve that you can't (or won't)?

Startup founders are typically neurodivergent already

I'm hella aspergers (now ASD), how much are you offering? lol