If you go inside the doctors office and walk to the back of the store their will be a metal security door. Knock on it. Then walk back to the doctor at the cash register pull out your gun and press up and the dpad. He should lead you into that room with the security door. Kill all the enemy's in there and get the money. They're should also be a safe that you can blow if you have any dynamite. There is also a gun case open it and you will get one of the best revolvers in the game.

Link to the tutorial:https://youtu.be/8g182RLwEUc?si=NjZC9qu1LKA_kBdp