Instructions
Kill enemies. All the enemies throughout "Epic Mickey" drop items at random. The larger Blotlings and Beetleworx have the best chance of dropping Sketches. Look around on the ground after you defeat a wave of enemies, and you may find a Sketch.
Open chests. Every level is packed with hidden chests and breakable items that contain Gremlins, sketches, paint and thinner refills and other items. Explore each area thoroughly, so you don't miss out on any hidden items.
Visit shops. The shops on Mean Street and in Bog Easy have lots of items for sale, including Sketches. If you ever find yourself low on Sketches, head to either of these areas to restock -- that is, if you can't find any by killing enemies or exploring.