This has come up a number of times. exoticwoods.com sells lumber in 1/8" slices & carries all kinds of wood. If you want an oddball size like 3/16", or if you already have some 1/4" wood that you want to shave down, the Stanley Pocket Plane is a fantastic product. The replaceable blade looks more like a rasp than a plane, and cuts cheese grater type slivers instead of big curls like a block plane. But it can hog off a lot of wood with minimal effort, & does not need a lot of force to cut. The body of the tool is long enough that it won't follow the whoop-de-doos in the wood if you've sliced it by hand. Most hardware stores carry the pocket plane for around $8.