This question is set in a story where the Yellowstone supervolcano exploded 40 years ago (possibly others, doing research on it) during "The Mining Incident," and following this incident the Midwest and the western edges of Appalachia and the Ozarks are abandoned. A dystopia begins, and citizens are forbidden to enter these regions.
The story begins with a protagonist who escapes into this region, and who will return to challenge the society back home. While here, he finds a partially-buried university, which was evacuated and left as-is at the time of the Mining Incident. Some buildings will be buried under a foot of ash and dirt, some will be partially exposed and the protagonist could climb in through a window.
For the purposes of the question, we're not going to worry about how this amount of ash fell here. We are also going to assume that
- No windows were broken during the blast (but could break under rain/hail later)
- No humans have been here since evacuation, with the only possible exception being a guide telling the protagonist to go here (he has taken nothing)
- Animal life returned about 30 years ago.
The character has a particular costume design, and I'm looking at the question of how he comes across the clothing for this costume. There are other possibilities, but like this one in particular as the discovery/donning the clothing is symbolic of later actions in the story.
Given the conditions above, would the protagonist find wearable clothing? He may find a university store, a laboratory supply shop with new clothing in original (plastic wrapped) packaging, and clothing abandoned in offices and dormitories. Among many other goods you would expect to find in this setting.
One other item he may find is a time capsule that, coincidentally, was meant to be opened the year he arrives.
My current thought
My current thought is that wrapped/packaged clothing that isn't in regular direct contact with water would be OK, most anything else would likely have been ruined by humidity and animals.