You didn't say what kind of steel, beyond "stainless or carbon". Since that description covers all steel, it is not much help. If you just need a piece for some construction project, any of the on-line metal suppliers can do it ( Speedy Metals, Online Metals,Ebay, etc.) If you want blade steel of a specific type (5160,ATS34,etc.) then Admiral Steel or Crucible, Or Kelly Cruples would be good sources.
As said, if you don't need any specific steel, try a scrap yard.
Stacy
Stacy