You likely, at a minimum, need to replace the regulator on the board. However, there's no guarantee that that's all you fried, so here's how it's likely to go:
- you order and receive a replacement regulator. $, plus shipping $
- you successfully swap the regulator, and try the board. No luck.
- you order and receive a replacement ATMEGA328. $, plus shipping $
- you successfully swap the CPU, and try the board. No luck.
Sense a pattern yet? Buy a new Uno, hang the old one on the wall (clearly marked, so you never try it again).