Your local shop? If you don't have a local shop, any competent shop will be able to make exactly what you need and drop it in a box with a stamp. It doesn't look like Duncan or Dimarzio have single humbucker guards.
So why single humbucker? Why not go for HH and use one or the other as you choose? If you want the easiest way to go take a look at this one from GC. I just installed one on my Strat. No solder, completely plug and play. Comes in black or white.