I'd recommend actively posting about your idea/project/product on Twitter, HN, LinkedIn, Reddit... Introduce what's great about it, where's the challenge and at the end mention that you're looking for a design co-founder and ask your followers to share. I'm a product designer and if I hear of an opportunity to be involved in building a product that would capture my interest, I'd reach out.
This is what such a tweet might look like:
"We're building [product] for [audience] and our goal is to become [vision]. We are busy with development and looking for a designer to help us with [tasks you need help with]. Please share. I'm open to DMs."