You can use the same websites you use on your computer. Just use an app that can manage downloads instead of mobile Safari, such as this one.
The drawback to using this method instead of iTunes is that the songs don't import into the iPod app on their own. You can only play them from the app you used to download them until you sync them. You'll have to use iTunes on the computer to move them from the app to the desktop and then into iTunes from there. They will show up in the iPod app after you sync.