Check the “business” section of your local and nearby cities Craigslist … if they post a picture, it helps. Also Ebay can be an option, but shipping costs have to be factored into the process. In either case, if the seats are upholstered, don’t be too worried about the color … just contact a local upholsterer first to get ballpark costs for whatever color or fabric you want. Finally, Google restaurant fixture resellers in your area. If all else fails, new chairs are available online at some pretty reasonable prices, and some of those sellers will pay shipping for an order of 30 chairs.
Not sure if you have a restaurant supply auction in your area but if you do they constantly have that kind of stuff coming through their warehouse. craigslist is also ok.