The first thing you need to know is the diameter of the portion of the fork tube that the clip-on clamp to. Since you can have different diameters on the same tube its important to know the diameter where the clip-on clamps. Measure this in metric. This measurement should be close to the full millimetre measurement. Here are some examples of some bikes I have put clip-ons on "Examples 49.90 is 50mm, 53.88 is 54mm, and 52.02 is 52mm". When you find your Diameter your looking for, do an Internet search for Your diameter and clip-on. This should bring up a few options. Woodcraft clip-ons are my personal favourite, but Renthal and Vortex also has some quality stuff.
Don't Forget that the Diameter you want is the same diameter your current clip-ons are clamped to.
I hope this helps.