i have been looking for the same. email marshall at endless bike co., he has a 16t left. a proto that he is willing to sell cheap.
i am personally look for something in 18t. i may try to drill a 18t shimano dx cog or something.
two issues that i am not sure of;
is the disc mount on a hub tough enough to handle a fixie cog?
and, chainline? will a cog mounted to a disc mount rear hub have a 50mm or so chainline?
i will report back later if i do try to drill on out of a dx cog. i think it may be the easiest way for me to try fixed in the dirt.