There are many buffet restaurants in Tokyo and food from all around the world. If you are looking for something Japanese, I have been to Budo no mori Godanya near Tokyo Station. Their food was quite nice.
Visit the 9th floor of the shopping building called LUMINE in Ikebukuro. The six restaurants there are all buffet restaurants.
Several hotels have buffet resutaurants. If you want to eat specialized meal,you can go to shisu buffet (ginza maguroya in tsukiji),pizza buffet(pizza hat),sweets buffet (bitter sweets buffets), yakiniku buffet(sutaminataro).