The parts are custom manufactured in England by a leading international aftermarket motorcycle gear and sprocket engineering company with over 70 world champions riding its products. Blanks are machined from solid EN36 chromoly steel stock. Teeth are precision milled and the cog drilled to very tight tolerances before undergoing heat treatment.
You can do something similar with a BMX sprocket and a pillar drill, but you're unlikely to get quite as neat a result. Take a look at www.londonfixiebike.co.uk and see what you think.