MAKE YOUR OWN Sanding boards!
(Commercial) sanding boards are usually too rough for good rendering points on graphite or pencils. You need: Double-faced tape. Carpet tape works well.
A piece (or several) of hard, waterproof material (not glass) – metal or plastic is good, old kitchen spatula or just get a piece of plastic from the hardware store, 2″x8″ or thereabout.
Wet or dry sandpaper – Hardware or building supply stores carry this (usually black in color). I use 200 grit – 320 grit and 400 grit. Cut into 2″ strips and put double face tape on back and affix to your plastic strip. (I use carpet tape, it holds good, removes easily and can take the water when washing the sandpaper out).
These sanding boards are then washable and you can keep both your work area and hands cleaner. I wash mine and dry with a hair dryer about every half hour when sketching.