I've written a few thank-yous to people who assisted, or to their managers,
but I don't have any boilerplate or template for that.  You should be as
specific as possible, about what that person's contribution was, and why you
appreciate their help.

On the other hand, if this is an endorsement for publication, there may be
some other rules/suggestions that apply.

I'd search for "Letter of Appreciation" either on the Internet in general,
or in a newsgroup that covers business correspndence.

Larry Linson
Microsoft Access MVP