All sorts of magnetic vent covers on Amazon, eBay, etc. if your object is to close off the duct. I really don't understand when you say they look "exactly like the registers that you are placing them on" unless you're talking about the color!
Or are you talking about a vent itself with adjustable louvers? They are available at most home stores (Lowes, Home Depot, etc.) or Ace hardware, and of course online as well.