Wrong forum for this you should be in "The lounge" but you simply call anyone you think would be interested in advertising at your show. Local auto parts stores, body shops that do hot rods, maybe even the wrecking yards. That and any rod or hot rod supplier you can simply call like Brizios, Speedway and the like, you never know what you will get.
I did exactly what you are doing once for a very large show here in California and got all kinds of stuff! I sent a letter to Meguires wax company and they sent me a gift box worth about $100 for us to put in an auction. You never know until you ask.
People put tools like little screw drivers, or key chains. Calendars, pens, some shops like where I work have beautiful cloth shopping bags, registration and insurance holders for your glove box, day planners, we give away thousands of them a year personally and give away boxes of them to thinks just as you are doing.
Just start calling around and you will get all kinds of stuff. Be ready to tell them how many cars are going to be there, that is the selling point as well as gives them a number of "bag stuffers" they need.