The library window has a button "Materials". You have to click that. This Library button is only visible while you are in the Material room.

I don't know if there are actual materials for those items. Quite often, people put materials into "Pose" files instead. These are called "Mat Pose" files. (They are a kludge. For a long time it was the only way to put a "set" of materials into the library as a one-click solution.) A lot of the default content is in the library as poses instead of materials.

Nowadays, we have "Material Collections". But a lot of people are intimidated/confused by the Material room. Because the programmers made the unwise choice to only show the materials library if you are in the material room, and that the vast majority of Poser users who are afraid of the material room, content providers tend to continue publishing material collections as Mat Pose files instead of as materials.

This general theme is one of the most amusing aspects of Poser in my opinion.

"Where are the hairs?" answer: props
"What about XYZ hair - I can't find it?" answer: oh, that one is in figures
"Where are the materials?" answer: poses
"Are custom characters in Characters?" answer: no, poses

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)