Master's thesis - lack of evidence, possibly poor topic choice
Don't panic. It's quite normal for the direction of the research to shift a bit after the initial proposal. It's common to find out that for one reason or another it's not feasible to do exactly what was proposed. After all, if you knew exactly how to do it in a way that's guaranteed to work, then it wouldn't be research!

Unless your funding is tied to a specific project, you can probably broaden the scope or make other adjustments. I'm sure your advisors will have some specific suggestions for you, but in the meantime, think about what you could do with the types of materials that you have been able to find.