The most convenient way is to fly into Indianapolis International Airport. Other choices are Chicago O'hare and Midway Airports.
Indianapolis International Airport (IND)
1 hr 15 min drive to Purdue.
Frequent Limo Service: Lafayette Limo Service (to/from Purdue): http://www.lafayettelimo.com/
Reservations recommended. Buying a round trip fare is cheaper than two one-ways.
Chicago O'Hare International Airport (ORD)
2 hr 30 min drive to Purdue.
Express Air Coach (to/from Purdue): http://www.expressaircoach.com/
Chicago Midway International Airport (MDW)
2 hr drive to Purdue.
Purdue visitor information: http://www.purdue.edu/visit/navigating/directions.php