Yes, a crank arm extractor is mandatory if you want to reuse the crankarm. You can buy extractors from LBS or online and are a very common item. Make sure you get the right extractor for the crank interface: square taper, ISIS, Octalink...
Here is an example of one for $7, that works on the above three crank arm types. It's not "shop quality" or bears a fancy name, but seems to be perfect for the weekend mechanic.:
http://www.pricepoint.com/detail/145...ank-Puller.htm