Any GPS antenna will work. The only stipulation is that it has a GT5 connector, that is what it needs to plug into the Magneti Marelli NIT. Most of the GPS antenna's that I found locally (usually at Marine stores), used BNC type connectors.
And yes, what I did was actually ordered the GPS mount from Maserati (its like a whole $17.00, part number is: 382300605). There is a screw under that grill that you take off, mount the GPS mount, and put the screw back in. The GPS antenna has a magnet built into it, so it grabs right onto the mount and won't move at all.
The antenna's wire is about 20' long, so there is plenty of length to run it down through the console and back to the NIT under the arm rest.
Hope that helps.