The Federal Aviation Administration's website has a search function for assigned or reserved N-numbers as well as available N-numbers. Aircraft owners can also request an N-number reservation at the FAA site.
The N-number is part of the international aircraft identification system for civil aircraft. Began immediately after World War I, the system uses the first letter of the identifier to signify national origin. The system assigned the letter "N" to the United States. The United States limits its registration numbers to no more than five characters after the letter "N." Those characters can be all numbers, or one to four numbers and one letter, or one to three numbers and two letters. The FAA does not allow the use of the letters "I" and "O" because of their similarity to a number. Also, a zero cannot immediately follow the beginning "N," and only the FAA may use the numbers N1 through N99.
When choosing an N-number for a civilian aircraft, owners can use the search feature on the FAA website to determine if the number they want is available. If the number is taken, the website will display details about the aircraft including aircraft manufacturer, type, year manufactured and engine type. The listing will also display to whom the aircraft is registered and the airworthiness of the aircraft. Owners can request the reservation of an available N-number from the FAA. The request does not guarantee the owner will receive that number though. The FAA processes requests in the order received, and it is possible that more than one owner can request the same number.