I ask applicants for a Github link, but it's not nearly as big a deal as you might think. Some people have interesting work on github. Others don't, but have something else to show instead. I'll look at whatever a candidate sends over (within reason).
And open source contributions are a lot easier to make than you might expect, when you consider non-code contributions. Opening an issue on github for an open-source library is a great signal. A well-written bug ticket tells me:
1 - You write enough unique software to encounter bugs in other developers' work (i.e., you're not just duplicating the same CRUD work everyone else is)
2 - You understand how to identify common tasks in your application that should be delegated to a library (and don't try to just re-write everything yourself)
3 - The details in the bug ticket show that you understand what information is relevant to solving a problem and can communicate with other developers.
Those sorts of contributions should be possible in most work environments, as part of your normal job responsibilities.