one of them is in the break room

one is in the office on the ground floor, on the table

another is in the men bathroom

then, on the ground floor, in the storage boxes room you can find it on top of a shelf

you can find one on the second floor in one of the many employee computers, mostly in the second area.
Lastly, you can find it in the CEO office where the chicken is.
Keep in mind that for sandbox these places to work you will need to have sv_Cheats 1 activated and then type "ent_fire case pickrandom" in the console, when done a keycard will spawn in one of said locations. In TTT one will already spawn.
I hope this was helpful.
Also if you find one you can save it as a dupe and use it whenever you like, it works.