Y combinator application
Does Y combinator accept applications from foreign applicants with an idea
Do we have to start a company in the US for you to invest in us?
Yes, but that's usually not a problem. It's easy for foreign nationals to start US companies (much easier than remaining here physically), and investors and acquirers prefer them.
Do we have to be US citizens?
No, as long as you can get here for at least three months. We've funded many startups founded by non-citizens.
Can you get us visas?
No, sorry, we don't do that. You'll have to figure out visas for yourself. If you know people from previous YC-funded companies who came from outside the US, we suggest you ask them for advice. They understand the options better than we do.
You can alway apply late ad get going on your idea and if you are not accepted apply on time for the next cycle with development of your idea already under way.
Although, you might need a bit more than an "idea".