Tell me where I can find sample of thank you letters which I have to send to my good customers

Sample Thank You Letters written by professionals (http://www.letterrep.com/showcatletters.php?catID=163)
How To Write Thank You Letters With Class (http://ezinearticles.com/?How-To-Write-Thank-You-Letters-With-Class&id=15958)
Appreciation Letters (http://www.writeexpress.com/appreciation.htm)

Of course, you don't need a template to write something sincere.

YOU HAVE TO... SOUNDS LIKE YOU DON'T KNOW MUCH ABOUT SALES. You should WANT TO send them to ALL YOUR CUSTOMERS, small or large. There are plenty of printing companies that have pre-printed thank you cards etc. try some of them. Look for them online. Just Google Pre-Printed thank you cards.