Try http://www.MP3converter.com
It lets you go from MP3 to WAV and vice versa. There's a free trial version that lets you make about 10 conversions. The actual version is like $10.
I second this. It's very easy to use, is very flexible, and performs quite well. I believe it uses the Lame program to actually convert to mp3. And Lame is well known and is very stable and produces quality output.