After much research, I found the cheapest 4 day hopper pass at my local Disney Store. There may be cheaper places to get Park Hopper Plus passes and 5 Day Park Hoppers, such as Ticketmania and some of the ticket brokers mentioned at mousesavers.com. Keep in mind when ordering tickets by mail that you often have to pay shipping fees.
I also would recommend NOT picking up tickets from Will Call at the park. I had a bad experience with that last summer. I also would recommend getting tickets asap if you know that you are going for sure. The ticket prices seem to go up at least once a year.