I put that info on the topic 19" rims. Be careful since my dealer said they have had a lot of trouble with vibration on aftermarket wheels. The best ones seem to be HRE. When I talked to HRE, they said to make sure the wheels are hub centric. They generally install 19" rims. They have also installed a 20" set but the tires looked very thin and didn't fit the car well in my opinion. The offsets given to me by HRE were 49mm on 19x8.5 in front and 43mm on 19x10 in the rear. Call HRE for the bolt pattern, I believe it is a common pattern but don't recall what it was. My dealer also did not know the offsets. I posted 2 photos of 19" HRE wheels on a spider that they sent me on www.maseraticc.com. Personally, I think other HRE wheel models look better than the ones they sent me and I prefer the R versions with hidden hardware (800 series). Good luck.
Have you called Tire Rack? They usually know that information. If you are anywhere near South Bend, IN and stop by they will also give you a deal if your car isn't on their website. They photo your car in their blue room so they can then show you what different wheels look like on the car.