The Best Places to Buy stainless steel history Online and In-Store
A good hardware store will have stainless steel tubing. A lot of the local mom and pop stores have gone out of business but if you have one try there. Otherwise try a Home Depot or Lowes.
This metal was usable and did what I needed to do with it however it was cut with a power shear and not very well. One side of the edge was mashed in and the other was slightly extruded out. As a result the material was not perfectly flat. If you are going to machine this and refinish it will probably work for you as it did for me. If you are expecting a dimensionally perfect flat piece of material it may not.