As a Canadian, I'd suspect entering the US under a TN visa (Free trade agreement if you have an applicable university degree or 10 years experience) would probably do it. Anybody have experience with that when attending YC?
For TN you need a job offer. Specifically, when you try to enter the US on the TN status (it's not a visa technically), you'll be asked to show an offer letter from a US company that clearly states your intended role and salary.
Just another entrepreneur here, but I'm a Canadian citizen and also need a VISA.
I've received excellent help from Julie Pearl's firm: https://www.immigrationlaw.com/
They work with plenty of startup founders and I was recommended her through fellow immigrant founders. :) Hope this helps!