Ask HN: I've an idea but not all skills required, what do I do?

I've an idea for app that I think could be really useful but while i can do backend side of things I need someone who can do Android and iPhone. I could pay someone but I want someone whom I can throw ideas back and forth.. I had one friend in mind he said no. What do you guys do in this situation.

It's very simple thing, if I don't get anyone I'll just make web version but probably will have to change Target users.

But I'm still curious if you don't have friends who are interested how do you find a cofounder.