well since you did specify wether you're looking for a place online or on your phone, I'm going to give you an app for your phone. There's a great app called ZEDGE and it has thousands of free wallpapers, ringtones, alerts and so on.
I agree with ray. Myxer is the ONLY place I know has a large selection in wallpaper (and ringtones, alerts, etc.) and is a SAFE website. I use it all the time. Most of them are free....actually a LOT of them are free, as well as the ringtones, and other alerts, screen savers, etc.