Below is the site I have been using for years for high res backgrounds for most any device capable of having a background. Up until now (well, shortly from now) the site was fee based but soon the site's mobile background content will be opened up to the public for free.
http://www.digitalblasphemy.com
You can select your resolution for the mobile device of your choice, and if you are unsure of your resolution, the more popular models are listed in the drop down.
I'll put the requisite "I searched for this and hope this is in the right area of the forum" disclaimer here.
EDIT: The mobile backgrounds are now free at the above site - available for nearly every mobile device