Both are nice. However how do I actually get those images onto the phone?
I don't use it, but I just checked out interfacelift.com and, after you find a picture you want, select the WP resolution (480x800) and click the download button, it'll load the picture, tap and hold on it, and then select "save picture." The picture will be in your Saved Pictures album in the picture hub.
As for where I go, I Bing and then make my own. Then again, I don't change my stuff a lot, so I'm probably not a good person to ask for sources.