I think it's important that you know the other person for a good amount of time (a few years, maybe?) before starting a company. Ideally, you have worked with this person closely.
The best answer to your question is: At work.
If you think somebody might be a good potential co-founder that you know, ask them. Don't assume that they are not interested in building something with you. You never know what people think, and even if they say no today, they might come back around in the future.
Comments
I think it's important that you know the other person for a good amount of time (a few years, maybe?) before starting a company. Ideally, you have worked with this person closely.
The best answer to your question is: At work.
If you think somebody might be a good potential co-founder that you know, ask them. Don't assume that they are not interested in building something with you. You never know what people think, and even if they say no today, they might come back around in the future.
Good luck!