How does the super mutants have access to all those weapons? I mean, they are all armed to their teeth: Miniguns, Laser Rifles etc. As far as I know they used to be normal people and they got infected with FEV (Force Evolutionary Virus), were they in fact part of some government program or they were military and got infected while sitting at guns stockpiles?
Super Mutants are a result of experimentation and exposure to the FEV virus. The US government developed this virus to try and combat the the Chinese threat of biological warefare originally to make the subject immune to all diseases. Once they found out during testing, that it made them massive in size and extremely muscular they moved research to a military base called Mariposa.
The master started of as a human called Richard Moreau/Grey, when he stumbled upon the base with a team(including good old harold), they were all killed by 'monsters'(apart from him and harold) and he was thrown into a FEV vat. Thats how he transformed into the Master, he started to absorb the other creatures and capturing humans to expose them to the FEV virus to create super mutants.
Just so happens he was sat on top of a military base! With lots of weapons at hand. After the'army' was equipped, with their toughness and strength they could easily overrun other heavily armed vaults/towns looking for more humans and supplies. In fallout 1 if you visit the military base, the base is still in relatively good shape and stocked to the brim with weaponary. Some I am guessing aquired and some that was already in the base before the master!
EDIT: As mentioned by @Robotnik, not all Super Mutants are well armed. Across the games, you tend to see Super Mutants, like humans, in groups or 'Factions' some being very well armed and intelligent/organised with others smashing skulls in with pool cues for the fun of it. :)