Hi Have the same kinda requirements and was looking for a USB Xmod in UK but couldn't find one, until I came across trustedgoods dot com. I'm not connected to them, but they are in UK and have these units in stock. I even got hold of Creative Europe and they told me they do not have anything suitable and the Xmod is only available overseas.
If you don't get any luck with them then try lindy dot co dot uk - they also do a USB sound card that will work with the MAC.
My particular requirement is to connect some external speakers to my 24" LED screen that I use with my MacBook Pro. Even though there is a sound card and speakers built into the screen, they are not great by any means (see all the screen reviews), and the screen doesn't have a audio out / mic jacks. The sound card in the MacBook is also just good enough for the internal speakers.
I have also tried the USB X-Fi GO and although it works great from a sound point of view, it completely trashes things like spotlight and causes a load of other weird issues - most notable things to stop working were iTunes, Time Machine, and it often would not even restart or shutdown until I pulled the X-Fi GO out the USB port. In a word - X-Fi GO is not suitable for MAC.
HTH.