You can find here crackberry.com/free-wallpaper or you may Google it and trawl other websites. But i would recommend you to download from crackberry.com.
Oh friend here is a website "blingcheese.com/wallpaper/6/blackberry+wallpaper.htm", just copy and past on your browser. I hope it will be helpful for you. If you want more websites, simply search on google there is huge list.