There are 6 inventory upgrades over the course of the game, however you won't find the final one until fairly late on.
Where to get all of the inventory upgrades are (roughly in order of collection).
UPDATE: The locations listed below are for the first run of each character. The locations are slightly different in the second run.
Police Station:
1 - West Office 1F - inside the safe - Left 9, Right 15, Left 7
2 - West Storage Room 3F - on a table near the C4
3 - Underground Facility Operations Room - in the safe room after the first boss fight, open the locker to the right of the typewriter
4 - Safety Deposit Room 1F - inside locker 203. You will need both spare keys from the two portable safes for this though. First portable safe is in the Men's Locker Room 2F, the second is in the Linen Room 2F and requires the Diamond key to unlock the door to this room
Sewers:
5 - Sewers (Upper) Workroom - use the T-Bar Handle to open the door in the Lower Waterway, leading to the Workroom Lift. Use the lift to go up and the pouch on the table
Underground Lab (NEST):
6 - Laboratory B1 - Nap Room - use the signal modulator in the Nap Room to turn the power back on, the pouch will be in one of the sleep pods.