In many papers dealing with topological Hochschild homology, the original unpublished manuscripts by Bökstedt are cited. To name one example, in McClure and Staffeldt's On the topological Hochschild homology of $bu$, 1989, we can see

Topological Hochschild homology (Preprint, 1987)

Topological Hochschild homology of $\mathbb Z$ and $\mathbb Z/p$ (Preprint, 1987)

The natural transformation form $K(\mathbb Z)$ to $THH(\mathbb Z)$ (Preprint, 1987).

On Angeltveit & Rognes' Hopf algebra structure on topological Hochschild homology only the first two are cited, but are attributed to "ca. 1986" and to the University of Bielefeld.

On Vogt's Introduction to algebra over "brave new rings", again, only the first two appear, and attributed to 1985. And the list could go on.

Anyway, the question is, where could one get hold of these preprints?