The Toothbrush spawns on ANY difficulty.
The spawn chance is 100%

Here is the Youtube Guide


So this is all the spawns i could find after hours of testing, however if you find any new locations post a screenshot and i will credit you for them at the bottom.

Outside Dumpster

Behind the couch on the far entrance


By the closed front door opposite the deal room behind some boxes


On some boxes at the back of the room with the counter and the cash register (Just turn around from the last image)

In back of the room where you drop the money bag for the deal

By the other entrance not the one with the sofas behind the bathroom door

Far left from the bathroom in the storage room leaning against the filing cabinet

By the fire exit behind the sofa

Turn around from the previous and go to the other end of the hall behind the filing cabinet

First door on your left as you come up the stairs, in the bathtub

In the opposite corner to the previous in the bathtub

First room on the immediate right from the previous underneath the bathtub