From the context of your post, it appears you're referring to private property, right? If so, why would you think you'd need a "permit" that some govt. agency would give you?
But if you meant public sites, very few, if any, locations have a "permit system". A few isolated cities in the entire USA, maybe a couple of counties (out of thousands of them), etc...
If you are skittish about rules/laws, then the way to do what lost-causes says ("
Learn the laws of your area") Is to look up the rules, laws, and codes, for
yourself. The reason I say this, is that I noticed you say you "
called several agencies". The danger of doing that, is that some clerk you call might interpret you question as a form of asking their permission (eg.: "Can I metal detect?"). And that simply leaves your question up for the whims of some desk-clerk to say "no", when maybe there's no real rule specifically saying you can't. Like if they morph something else to apply to your "pressing question", like .... don't disturb earthworms, or whatever. (when in fact, no one might ever have cared or given you a second look).
Better to look up laws/codes for yourself. Nearly every county, city, and state nowadays has web pages now. And from there, find a subsection of municipal codes, park rules, and so forth. If you see no prohibitions on metal detecting (ie.: silent on the issue), then presto, I guess it's not disallowed now is it?
