Here's a quick guide on how to find all the scavenger hunt items. I've used pictures and help from George Fosters guide on the same topic from thegamer.com:

https://www.thegamer.com/psychonauts-2-scavenger-hunt-collectibles-location-guide/

Notice!

You'll be able to get the quest from Norma as soon you get your new outfit from Sasha.

There are 16 total items on the list. Completing the scavenger hunt will unlock the ability to change clothes in Motherlobes toilets, which includes the suit, circus attire and original Raz outfit.

You won't be able to finish the hunt before you've unlocked the Projection ability from Cassie much later in the story and completed the game.

Rotten Sushi:

Cafeteria. In the buffet next to the lunch ladys station.


Psychonauts Name Tag:

Agents section. Next to Milla's office from Coach Oleanders "office" door.


Enemy Surveillance Drone:

Can only be obtained after learning the Projection ability.
Storage Closet. Open the door which can be interacted with Raz projection. Drone is sitting on the lockers.


Deck Of Cards:

Bowling alley. Grab the bowling ball with telekinesis and throw it down the alley few times. The deck of cards will eventually appear from the ball returner.


The Astronaut Ice Cream:

Central Nerve. Interact with the green-glowing computer in the middle of the room and see how the green light switches screens to another computer. Follow these green screens until you arrive in to a corner, where interacting with machine will pop out the ice cream.

Unexploded Bomb:

Near the entrance leading to Questionable Area. There's a campfire near the mine entrance -- light it with Pyrokinesis to create updraft. Ride the updraft with levitation and you'll see the Bomb sitting on the crane above.


Psitanium Knife:

In the cave near Lilli's garden. Lilli hangs out on the right side of the Quarry, and right near where you'll see her, you'll see an entrance to a cave. Cave contains some easy platforming to get to the Knife.

Agent Orientation Laserdisc:

Otto's Lab. Behind Otto, there's an tiny area protected by an electric floor. Use levitation ball to ride over it and you'll find the Disc.

Can of Corn:

Sassyclops Cave. Go into the cave and light the first campfire with pyrokinesis to create updraft. Ride the updraft with levitation to get on the ledge above, which containst the Can.


Switchblade Hatchet:

You'll need the Time Bubble ability from Brain in a Jar level. Right outside Aquato camp. There's a water wheel spinning really fast. After using Time Bubble on it, climb it to the roof, where you'll find the Hatchet.


Human Skull:
Funicular. Start up the funicular by locating all the cogs from nearby. The skull will be on the left when riding the funicular.


Novelty Mug:

You'll need Projection ability to reach this one. Lumberjack Diner. Get Projection Raz to open the door next to Sam and the Mug will be sitting on the shelves.

Mini Murder Bug Bot:

The house, where Ford hangs around. Climb the ladder in the house and burn the thorns with pyrokinesis. The Bug Bot will be on the balcony outside.


Viking Helmet:

Past the Heptadome. There's a camper van near a stage, which is Helmut's real Feelmobile. Behind this van is the Helmet.


Beehive Shaped Like My Phone:

Behind Cassie's house. Use the levitation ball to jump around the side of the house to reach the back. You'll see the phone there.


Signed Copy of Mindswarm:

Inside Cassie's Hive. This is a special one. Just go up to Cassie and talk to her, and ask her to sign Raz's copy of Mindswarm. You'll gain the Copy this way.

I hope that you found this guide helpful. I tried to make it as quick and simple as possible.

The real credit belongs to George Foster, who wrote the original article.

Now get to hunting!