That answer all depends on if you are willing to invest money or not. There are website who provide you with free content, but there's always a catch and these wallpapers are not going to be original.

If you want original wallpapers, then you are going to have to invest some money into it. The best way to get original content is to buy a high end digital camera. Take pictures of anything you want to and make them into a wallpapers using software like photoshop. Then make sure you copyright them so no one will steal them from you.

So basically, if you want original, you are going to have to spend money.