This depends upon which company you use. There are a number of clubs that specialise in booking vacation "last minute" vacations. Some will let you book and take off within a 48hr period.
You can book tickets for last minute flights at several places. You can book right online at the airline website or in person at the terminal. You can also buy tickets at places such as travelocity.
A great vacation website if you are looking for last minute deals is: http://www.tripgo.ca/last-minute-deals I normally use it just because I can find deals that are not on other travel sites! I...