Track cranks are available but they are expensive and used a 144BCD chainring. I think most road fix conversions go with a road crankset with one chainring and a set of single ring stack bolts. If you are building a new ride, that will probably be your cheapest option. But if I were building a really nice fix ride, I'd go whole hog as use either the DuraAce, Campy, or Sugino75 track crankset.

Dave