There is a guy around Saskatoon who makes them and sells them on e-bay, a search under Tokarev or SVT-40 may work. A little $2.99 crescent wrench works just fine though if you're careful. I think Numrich in the states sells repros too. Originals turn up on e-bay and gunbroker too from time to time.
Edit: On E-bay search for seller tolya7777, it's a repro but it works fine. Originals are a little more money and a little harder to find I think.