metal mole, is the fairgrounds you have in mind private? Or fenced? I don't know why a fairgrounds would be any different than a school yard, or public park or beach, when it comes to "permission" issues. If it is a public fair-grounds, and not closed to the public (ie.: gates only opened for when there's an activity), maybe you can just go. I mean, of course, use discretion if there is a caretaker's premises right there and you're a sore thumb in front of him. Ie.: don't be a nuisance, or whatever, granted. But if it's set up just like a regular park type grounds ......
Also be aware that fairgrounds were the first places, back in the day, when md'ing was getting popular in the 1970s, that got pulverised first. Not that "anyone gets it all", and of course, there's always new clad replenishing it, but....... some fairgrounds are going to be stingy and worked hard of all easy gimmees, when it comes to silver/old coin hunting. Hopefully though, yours is virgin and never been hunted