Ok, so i have played the heck out of this game, but i still haven't found a fatman! Is there a place i could go to buy it and buy ammo for it? Also i found the Grenade machine gun but i cant find ammo for it. Gun runners has no ammo in stock. Any were else i could go? Thanks

Well, after doing all the Brotherhood quests, I was able to buy a Fatman from the BoS.

I have been able to find some on my own but the wiki will save me time writting the locations out. http://fallout.wikia.com/wiki/Fat_Man_(Fallout:_New_Vegas)

I found mine in pretty lousy condition in the Quarry Junction guarded by a dump load of deathclaws, don't recommend venturing too far in their unless your a really good sniper.

This. Though after visiting 180+ locations i have only found 4 or 5 rounds for the stupid thing.