Find a simple black robe/cloak at a thrift shop or even at a discount store with the Halloween costumes. Another option is to see if you can find an old graduation gown!
Most importantly don't forget a pair of glasses with tape over the nose. If you cannot find a pair, why not print out these.
Looking for more costume ideas and Halloween projects? Check out these easy costume ideas! You can also browse through my other Halloween articles and find many Halloween crafts, games, recipes, and other activities.