Search Harder

Usually, this involves taking a more oblique approach. Broaden your keywords, look to see if the author or publisher has any correspondence or papers archived at universities, that kind of thing. In the case of something like the Hitler diaries, your scope of "primary" documentation might be different because of the nature of the deception involved - Konrad Kujau might not have kept development documents, Hugh Trevor-Roper and Gerd Heidemann might not have kept notes. But there might be interviews or pieces they wrote that reflect on the process of creation, sale, "authentication," and discovery. If this were for a thesis or doctorate, you might even track down relatives who might have surviving documentation, but that might be a bit out of scope for a history capstone.

Expand The Scope

While you might not have much luck regarding the creation of the documents themselves, they weren't released in a vacuum. There would be advertisements, editorials, analyses from the time. Language might be a barrier, but you could trace early doubts (or lack thereof) about the source of the Hitler diaries, historian responses, etc. While not primary sources on the creation of the diaries, they would still be primary sources of its reception.

Follow The Bibliographies

One of the best way to get "closer to the source" is to take the standard works on the subject and...see who they cite! Track down the sources in their bibliography, and see who they cite. It can take a while, but it is often valuable to go back and see what the original sources they're drawing from are saying, and in what context.

Of course if they don't cite anyone, that's a red flag in itself.