I've only found them when I pulled them off of old trash bikes, so no source for NEW ones AFAIK, but if somebody has a lead, please chime in.
I've used a lathe but I think you'll find there are limiting factors: if you can clamp the TOP of the stem in the chuck, it's easier to cut right down to the end, but you will have the bar-clamp spinning around in such a way that access is limited up high (watch out for your fingers!). And if you clamp it on the bottom, you still have that bar-clamp threatening your hands, plus you can't cut down the portion that's in the chuck (I ground mine with a wheel to finish the bottom off). Good luck to ya!