Helvie makes one and I have another one from a German mfg. called Mascot, but that may have been a vendor name. It came in a four piece chip and relief carving set.
http://www.internethobbies.com/mawocaknset.html
I searched the Helvie sites and I don't see this blade any more. However, as a matter of safety, I ground down the back of one side on each of these knives so only the convex on one is sharp and the concave on the other.
Here's another one available from Woodcraft as a single knife.
http://www.woodcraft.com/family.aspx?familyid=1789 check "H" on this page.
I also have a set of Whittlin' Jacks that has one of these and I painted the handle
RED so when I pick it up I know it's sharp on both edges.
Al