From The Vault
BB's are a type of ammunition in Fallout, Fallout 2 and Fallout 3.
[edit] Fallout/Fallout 2
Description: A package of BB's from before the war. In excellent condition. Stainless steel.
[edit] Fallout 3
BB's are a small guns ammunition found in Fallout 3. They are used in only one weapon, the BB Gun.
[edit] Locations
Amazingly for a type of ammo so worthless, BB's are probably the rarest type of ammo, although unlimited amounts of BBs can be bought through restocking merchants. You can buy them from Flak 'N Shrapnel's in Rivet City, Smiling Jack in the Evergreen Mills Bazaar, Tulip at Underworld and Chief Gustavo in Tenpenny Tower.
An unlimited amount of BBs can be obtained from your father at the "shooting range" put up by Jonas Palmer inside Vault 101 rooms visited during your 10th birthday party: still, you will not be able to obtain but 50 of them at your last tutorial part, when you are waken by Amata.