i have to buy it o where i can find it
There's always some MFCs at traders in San Francisco, you can exchange BB ammo you've crafted from junk, which you harvested in the same square as the city - you won't meet dangerous encounters there. (devs, please don't "fix" it).
Another way: you get EnergyExpert 1 and 2 for 1500+3000caps, buy Uranium ore in Broken Hills, carry it to the ghoul in the south , refine it, present numerous possibilities to kill you, survive the long trip to Mariposa Base, don't get killed by PKs, craft dem MFCs by yourself.
Which way is easier, is up to you.