where cann i find online actual complaint letters sent to restaurants or banks or any other service provider?

Answer 1

You would only find them if they were sent to a consumer watchdog group that actually posted them. I can't see a business actually posting negative comments about themselves to discourage customers.

Your best best is to google the name of the business + reviews & see if they have been reviewed anywhere.

Answer 2

I only have the sample for your reference:
http://www.consumeraction.gov/caw_problems_sample_complaint.shtml

http://www.businessballs.com/complaintsletters.htm