Maybe it's different in the USA, but in Canada, each bank either has an affiliated Merchant processing company, or a company that they have made a deal with. So if you go to a bank, they can give you contact info., but I believe it's unlikely that they will have equipment on site.
There is no company I can recommend, I would suggest that you just jump in and start calling and emailing companies. From my experience most have start up fees and contracts for 3 years...but Square has really been hurting their businesses, so there may be more options then there used to be.
Try searching the internet for "seasonal" credit card processor, to see if there may be a company that will meet your needs, and will allow you to turn off your account for part of the year. Even the big companies may allow you to set up a seasonal account. I use First Data and I turn off my account every year for 4-6 months, but I don't have a machine, so a return of the terminal is not a cost I have to worry about.
That way, once you are set up with them, you can use them each year for these problematic shows.
