70% is surprisingly low to me. I'm currently looking for startup roles, and it's genuinely difficult to find any that don't offer remote, to the point that I'm starting to seriously consider a career change.
At workatastartup.com, you can filter by "Not remote" jobs, but it's probably not as intuitive as it could be. We need to revamp some of our search filters accordingly. But until we have it, feel free to email me (ryan AT ycombinator DOT com) and I'll do what I can to help you find ones that might be a good fit.