I also recover and return items at no charge. 30 recoveries so far. I average 4-6 requests a month.
Posting here is not likely to help you get permissions. Look for local sites.
Nextdoor.com is a free national site but the postings are kept local. Look for local sites on Facebook.
The trick to successful marketing is repetition. That’s why you see the same firms advertising over and over. I post on my local sites at least once every 3 weeks. Sometimes it’s a reminder to leave jewelry at home instead of wearing to the beach. (Don’t worry, they still wear it). Sometimes I show an interesting find. people love those.
If you have a good story or find notify the local newspapers or magazines. Often they will do an article. I found one item that ended up with full page or double page articles in 4 newspapers and 5 magazines including ones in Scotland and the UK. Those led to a segment on the local tv channel.
Volunteer to give presentations. Scout troops, schools, museums, church groups and civic clubs all are looking for speakers. These always lead to permissions..Last year I spoke at 3 museums, a senior citizen’s center, retirement home, a scouting group and a cultural event.
Absolutely have good quality business cards. Hand them out everywhere you go. I just reordered 500. The previous 500 lasted me about 6 months.
Have a catchy phrase ornament and use it consistently. I use FREE METAL DETECTING SERVICES! Many people don’t remember my name but they all remember that.
I concentrate my efforts to the local historical areas. You’d be surprised how many multi million dollar homes, especially those 100-200 years old, that people call me to hunt. Recently I was permitted to hunt 4 state college campuses that nobody has been allowed to touch.