Kristof's Firefly pendant is inside a blue storage container in the coal mine during The Hunt section of Lakeside Resort. It belonged to Travis Kristof.
Braun's Firefly Pendant
Braun's Firefly pendant rests on a bench inside a gazebo next to the first cabin at the resort, in The Hunt section of Lakeside Resort. It belonged to Paul D. Braun.
Ellie's backpack
Ellie's backpack is an automatic artifact Joel picks up in the back of the butcher shop in Lakeside Resort. As he walks in, he grabs it from a low shelf, and it stays in his Artifacts menu from then on.
Joel & Sarah photo
The Joel & Sarah photo is a picture of Joel with his daughter, an automatic artifact in Ellie's backpack at the start of Lakeside Resort. You can view it from the Artifacts menu while playing as Ellie.
Lake resort map
The Lake resort map is an automatic artifact Joel gains in Lakeside Resort. It goes into his backpack right after he interrogates the two hunters in the Cabin Resort section, so you cannot miss it.