As an event organizers you think no big deal, its $500, but as a business owner it is now a couple hours spent moving machinery to the show with multiple employees, a couple hours spent bringing it back from the show with multiple employees, a couple hours spent preparing it for the show and again cleaning it up afterward, not to mention anythign damaged in the process and then you asking for $500 cash on top of the thousands that will need to be spent making this happen for your town festival quickly turns into several thousands in cash and $10s or 100s of thousands spent annually going to shows, with each additional show resulting in more and more asks for donations.

As a volunteer you obviously have not talked to enough businesses or you would realize that it is quite common for larger businesses to contribute a higher levels and skip the smaller shows because the smaller shows represent a huge burden to the business.  Publicly critizing them afterward is also in poor taste and will likely get you told 'NO' in the future too because you come off as unapperciative.

In the future I suggest changing your approach as the old saying of: "You catch more flies with honey than vinegar" applies here.  Had you had two requests: 1) for them to display some equipment at the tractor show and 2) possibly contributing some money or prizes for the show and then not making 1 conditional on 2 would have been far better for you since 1) they would have had a presence at the show, and 2) had they not contributed prizes their customers would likely have asked the dealership why the did not sponsor any prizes at the show making it easier to get contributions next year and you would have benefited from more equipment on display this year, which makes for a better show, that leads to larger turnouts in the future.  As the event organzier you need to start thinking "How will this make our show better?" and quick thinking "What's in it for us?"