Not really a buffet but they do an unlimited salad bar. There is one at the Centertainment and one at Meadowhall. You just need to buy a regular meal which starts from about £4.
Website for centertainment
Valley Centertainment, Broughton Lane, Sheffield, S9 2EP
01142 619 836
Website for Meadowhall.
Address: Unit 13,The Oasis,Meadowhall Centre, Sheffield S9 1EP
Phone: 0114 256 8248