The M66 Rocket Launcher can be found within the Dead Factory. A version of this weapon, equipped with infinite ammo, can be purchased through "The Mercenaries - Operation: Mad Jackal" minigame. the rocket launcher cannot be aimed upwards or downwards.
How to Obtain In Resident Evil 3: Nemesis
A few items are necessary to obtain the rocket launcher. The player will need the Facility Key as well as the keycard obtained after the fight with Nemesis in the BOW disposal room (the keycard is red, and is needed to exit the room before the timer expires).
While the player is in the lower levels of the Dead Factory, solving the puzzle involving the Water Sample, they can attach the plastic card onto the Facility Key into a machine nearby. Doing so activates the card, imprinting it with a barcode.
After the player obtains the keycard after the Nemesis battle, they should proceed to the Save room and then enter the connecting room containing the steam valve puzzle. There is a nearby elevator that can only be activated through the use of the keycard.
At the end of the hallway is a locker that can only be opened with the Facility Key containing the barcode, wherein the rocket launcher is stashed.
Infinite Ammo Version
The infinite ammo version can be purchased in The Mercenaries - Operation: Mad Jackal mode by earning enough points in this mini game. It costs $4000.
"M66 Rocket Launcher. An old but powerful weapon."