Aftermarket dashes are big bucks if you can find them. There are several companies offering slip covers as a cost-effective alternative, but I don't know that the fit is all that great. The one I saw installed had developed large gaps around the vents over time. Maybe it was a cheap knock-off, maybe it wasn't installed properly.
1A auto is a great source for molded carpet. I've bought one from them and my brother has bought a couple. The quality and fit are excellent. Your seats and interior moldings are very simple to remove... you should be able to install it in one afternoon. Here's the link for the non-hatchback Prizm:
http://www.1aauto.com/1A/Carpets/Geo...m/ACC1240/6831