See http://www.sawdustroad.com/sketchup for a full explanation; this selection of wood textures was created and made public by AKB. Unfortunately, SU8 no longer allows you to import materials in the form of .skm files, so I manually unpacked them and created this model to make all the material textures readily available.
To use: download this model, open it in SU. Window > Materials > Texture Palettes > Colors in Model. Purge Unused. Select List > Duplicate.... Name your new palette "akb Wood Textures". Remove any remaining colors that weren't part of this model.