Welcome from South Carolina !!!!
I would suggest starting out getting permissions from people you know, relatives, friends, co-workers, neighbors.
They are more likely to say yes since they know you and trust you.
Then after you exhaust those places, ask them for referrals, people they know that you don't know, then when you knock on those doors you can say something like, "your friend, Joe Shmoe, allowed me to detect his yard and thought you might be okay with detecting yours" .....someone might be more likely to give permission from a referral.
After all that you should be more comfortable and practiced asking permission and can ask total strangers.
NOTE: Always remember to take any trash you find with you, and replace your plugs carefully so as not to leave holes or mess up their yards otherwise they might not be too willing to give you a referral
In case you never saw this graphic before, it can be of help in digging proper plugs and replacing them -
View attachment 388875
View attachment 388876