Dick Blick art supplies is a fantastic place to pick up supplies, like Michael suggested. Finding paper with just a little bit of texture will help you create the orange-peal effect you're looking for.

There are different blending techniques you can use to help achieve this along with the texture of the paper. We can go into more detail on that regard if you were wanting to go that route. I'm afraid I lack any skills with drawing using photo-editing software, but in the sketching forum, there is a sticky thread called "Best of...sketching tips and tutorials" or something like that. In there, we have saved numerous posts to assist with all areas of sketching, including digital sketching. Carlos Hernandez comes to mind instantly for digital sketching and he'd be a great one to start with.

Stephen (our Darkstar here in the solar forum

) is also very good with digital sketching as I believe he adds the mottling with the computer.