Where can I find sample sales letters that persuade businesses to purchase advertising space in a newsletter?

I'm looking for sample sales letter(s) that persuade businesses to purchase advertising space in magazines, newsletters, bulletins, etc...The sales letter I'm writing would be sent to local merchants (service industries like restaurants, insurance companies, etc...) to persuade them to advertise in a local newsletter. I'm looking for some free compelling copywriting that can bolster my sales letter. Any help would be appreciated.