I’m a student in Canada and I want to do an internship in San Francisco, but nobody wants to hire from across the border. How can I make this happen?

This is definitely possible talking to my friends (also a student from Canada). Your best bet is to target bigger companies/startups as small startups in seed stages were not going to be able to afford to sponsor you.

Go for Facebook, Khan Academy, Google, Tesla, Palantir, Velocity, etc instead of trying to target small scale startups (as sponsoring a visa is expensive). Also good to ask up front whether they take Canadians and if they don't then don't waste your time!

Find a company with a Canadian founders. They will understand the pain, and also know the quality of your school