How to write a thank you letter to an wlite customer who visited us by finding some time within his busy schedule

Handwrite it in your best handwriting or type it on your company letterhead. Say what you have said here. Mention something about your company looks forward to doing business with him in the future.

Here are some ideas (for an automotive dealer, but of course change the words as you need to):

http://www.paulcummings.com/pdf/100%20Thank%20You%20Notes.pdf