Sometimes you can find carriers on Craigslist, but not very often.
If you can't find a used one, I would recommend ordering from KWcages. They have quality products, and since they are based in California, the shipping fees will be less than ordering from out of state.
You could save shipping charges altogether if they (or another company) will be a vendor at the show and you preorder a carrier to purchase the day of the event. You just need to figure out a way to get them to the show safely and cleanly. Cat carriers bedded with pine pellets topped by hay or straw would be an option, although there is still more risk that they will get soiled than if they were transported on a wire floor.