Do you know what wood you want? Some of the knife supply businesses will have wood for knife scales and give the measurements. Another place I've bought from (though it was in person) is Gilmer Woods in Portland, OR. They have a ton of different woods and take pictures of the individual pieces, so what you choose online is exactly what you'll get. I don't really buy scales, just blocks, but you might check with them. Do you have any specialty wood supply places near you? Woodcraft is a decent place to take a look, too. And yes, some files/sandpaper would get you down to the thickness you want pretty easily.
Cool idea-I look forward to seeing the finished product.
Jeremy