Make friends with other entrepreneurs. Impress them and ask them for intros to their investors. If you're good, and your product is good, they'll want to introduce you as it looks good for them.
I have so many founder friends in SF, but their investors expect them to focus and work solely on their own thing.
I think you might misunderstand what I'm saying. An introduction is an email. If you already knew that, then you either misunderstand how the game is played, or your friends won't introduce you because they don't believe in what you're building.