Warren's Firefly pendant is the only one you loot from an enemy: it drops from the bloater in the spore-filled lower level of the university dormitory in The University. It belonged to Joe Warren, a Firefly who became that bloater.
Joe Warren is a real winner of Naughty Dog's Firefly Pendant Contests.
Firefly's recorder
The Firefly's recorder is an automatic artifact in the university science building. Joel picks it up from a dead Firefly in a cutscene, so it cannot be missed, and you can play the whole message later from the Artifacts menu.
Free Radicals
Free Radicals, issue 7 of Savage Starlight, is on the rusty hood of a car up the hill behind you as The University begins.
Fungal X-rays
The Fungal X-rays are an optional artifact in the university science building. After you follow Ellie through the short tunnel, turn left and check the far table next to the window.
Griggs's Firefly Pendant
Griggs's Firefly pendant lies on a table in the last tent of a camp outside the science building, in the Go Big Horns section of The University. It belonged to Erik Griggs.