I know they have them at Enoteca -- anywhere else in Austin have good moules frites?
Olivia's, on S. Lamar. I've had this a couple times and always very tasty. Bonus: if you eat at the bar on Sundays or Mondays, chances are they are on the menu and 30% off!
Peche has moules frites for appetizer (awesome frites w/ your choice of aioli), but I think the moules disappointed.
I think the best way to have mussels is homemade. It's really easy and you can be creative with your ingredients. Here's a link to my recipe: