RadioShack lists
this TO-220 heatsink as well as
this one. But the stores seem to be cutting down on their stock of these sorts of things so you might be out of luck.
The regulator won't overheat immediately, giving you time to yoink the power if you think it's getting too hot. It's probably worth checking
Tagent's power supply calculator to estimate how much heat the thing will shed.
Good luck with the DAC. I've been following this project as a possible outboard DAC for my Roku Soundbridge.