My RS came with one... its attached with a bungee cord inside the frunk. I've found the gauge is useful for the front tires, but hard to get to on the back tire.
CONSUMER WARNING: Use of this Spyder may cause smiling, happiness, urge to ride, excessive wind in the face, desire to call in sick to work, or even extended vacations. Some riders have experienced double-takes, pointing, sudden camera flash, kids waving, and conversations with total strangers. Several instances of unplanned roadside visits with police officers have been reported, though are not common. If you experience any of these symptoms, contact your Spyder forum to discuss.