The Apple Airport Express is available from Apple for about $99. I also have the Apple Airport Extreme as the base wi-fi station, so this little unit has no problem extending the wireless network. I have never attempted to connect the Airport Express to a wi-fi router other than Apple, so others may know if it works or not.
I originally had these small devices to use a feature called AirPlay. They have an audio jack on them as well where you can hookup your stereo to them and stream music to various rooms of your house. This was the original purpose for the Airport Express.
It looks like they work very well with the uRad so far (exploiting the Ethernet jack and the USB for power).
Dave