First I wanted to see how National provides their confirmation numbers.
I went to Nationalcar.com and booked a reservation. It immediately gave me a rental car confirmation number.
I was also sent a confirmation email with the car rental reservation number on it.
Without this information, I would suggest that you go to nationalcar.com and click on the contact us link (top right hand corner of the homepage in pale green type),
Then under rental car reservation questions, click modify or cancel reservation.
Then fill out as much information as you have (cities, dates, times, etc for the reservation that you had booked) and request them to send you a reconfirmation email with your National reservation or confirmation number.
Hope this helps!
Pages from this site, relating to this topic: