First make sure you check classifieds on here, in the paper, and craigslist.org. I found this a few days ago
http://www.reefcentral.com/forums/showthread.php?s=&threadid=1442584
Bit of a drive for ya but you may be able to work out a deal worth your while.
If you don't want to buy used I would shop around the region at different pet stores / aquarium stores. Once you figure out what size you want ask them to give you a quote and don't be afraid to emphasize the fact that you are gathering quotes from other stores. Nearly every store will tell you they are the cheapest so make them prove it. Once you have a few quotes then you can negotiate some.
I would definitely get a tank predrilled with overflows and you may want to consider getting a 6ft tank, if you have the space, so you have six ft lighting already should you choose to upgrade one day.
One more thing, if you can find a store with a tank on display, wait a week or so and you will have a much better shot at getting a good deal. The main reason most stores don't get the larger setup is because they can take very long to sell, at least in your area, often they get sick of holding on to them and are willing to negotiate. Never be afraid to ask!
Petland in Iowa City often has larger tanks sitting around maybe give them a call and see what they have. (After your check Waterloo, CF, CR area)
Let me know how it works out for ya!