Sketchbooks are a great resource for artists and the practice of sketching is invaluable. Looking back on past skecthbooks and sketchbooks of others reveals amazing works and a wonderful progression of improvents over time.
Below I offer these sketchbook images free to download. Personally I like to import these images into my digital painting tablet software then I draw on a layer above. The results look similar to actual physical sketchbooks. Give it a try.