On PC only, access the console by pressing the key under Escape and left of '1'. Then type in player.additem <item base ID> <amount> <condition>.
Base ID's are the codes of items in the Fallout 3 world. Simply typing the name of the wanted item will not work. Search for the item you want on the internet. Try Googling them.
Word to the wise, you need to have The Pitt downloaded first.