All depends how large it needs to be. You will also need to know what type of stainless (301, 304, annealed, 1/4 1/2 3/4 full hard) and the thickness. What are you using it for?
Try any of the metal distributors in your area.
There are also probably some scrap and salvage yards in your area that are worth trying.
Metal distributors would probably be a little more knowledgable and honest about what they are selling you.
Sheet metal shops and Commercial Heating and Air Conditioning shops. Used for ductwork in Neonatal ICU's and other sensitive areas. If they don't have it in stock they can get it in their next shipment of materials (usually once a week or so). Their shop foreman can probably help you with thicknesses, polishability (is that a word? It is NOW!) and so forth.