When dealing with customers, you can clearly convey "no" without ever having to say the word. For example, iInstead of "no, we don't have that," try saying, "I certainly see how that would be useful and helpful, but I'm afraid we don't have that at this time," or "While we don't currently have what you are looking for, we appreciate knowing what you want so that we can continue to improve our service." Positive language, delivered correctly keeps the door open for future customer interactions, and the customer will not feel as though it was a waste of their time. 

This approach will always help the customer to feel heard. You want to acknowledge the effort and your gratitude for their time. Another way to win the customer over in a situation like this is to offer alternatives. "We don't have that, but what about this...?" Or, "Have you thought about trying this instead?" Again, this technique softens the "no" and strengthens both your relationship with your customers, as well as it positions your company's reputation as one who is willing to go the extra mile to assist.