Hot Cams shims -- as good as any, and good prices.
These vary in thickness a little more than OEM, but that can actually be helpful, as long as you have a micrometer. I have one from the 1.65 five-pack that measures at 1.663, which gives me a little elbow room if/when I need to move less than one full step -- or in cases where a step and a half would be ideal. I also like the fact that the size is stamped into the shim, instead of printed; often, the ink wears off partially or completely. I'm going to measure anyway, but it helps for organization too.