Dear

We have received your Purchase Order Number [number],
dated [date]. We regret to inform you that said order
cannot be filled at this time due to the following reason:

Should you care to discuss this matter further, please feel
free to contact me at the above number. We look forward to
your future orders and are ready to assist you in any way
possible.

Try Google Search to find what you are looking for!

Here is some random sample letters.