Best buy will do a fine job, and to an extent you can get a good deal if you catch a sale.... but installing a new headunit on this car is really easy, if you're up for a little DIY.
All you need is a Phillips head screwdriver, a wire crimper, and 1 hour.
1) On the flat plastic just under your radio, there are 2 screws (vertical).
2) Remove them and pop the gray panel from around the radio.
3) Surrounding the radio, there are 4 screws that hold the radio bracket in. Unscrew them and slide out the radio. You will need to slide out the antenna wire and the clip-in wiring harness.
4) Unscrew the bracket "L" pieces from the radio and install them on the new radio (without using the metal cage).
5) Using 12 electrical butt connectors or small wire nuts... create a wiring harness adapter. You'll need the Toyota harness (included with Crutchfield purchases). Wiring diagrams are available at http://www.installdr.com/Harnesses/Toyota-Wiring.asp
. The wiring for your new headunit will be included in the manual or install sheet.
6) Clip in the wiring harness adapter, along with the antenna wire. Slide the new unit into place.
7) Screw in the brackets and big gray trim panel.
That's it! As mentioned above, you'll just need a Toyota radio wiring harness and a new headunit. As Ecuador mentioned, Crutchfield is usually a great value, since you get all adapters and instructions included.
I'd highly recommend Pioneer and Clarion. Their budget headunits have nice radios and tend to be the most reliable.