These guys pretty consistently have the lowest prices for OEM parts:
http://www.****************************************************.com/parts/
However, you're going to need to have your new key programmed (along with your old key) by the dealer anyway at a cost of $60-$100. If you just want a spare, you can buy the key without the remote control head, but it'll still need to be programmed. If it's a key you're going to use on a regular basis, I'd get the remote head. Not having it is a PITA.
When I needed a spare key w/remote head I contacted Sunset for a price and quickly came to the conclusion that there really wasn't much to be saved by going mail order in this case. The bottom line is you're going to be in for a painful $250-$300 total.
There will be times when you'll want or need to buy from the dealer. IMO it's good to throw the parts guy a bone once in a while, so when you're in a bind (someday you will be) he'll hopefully go the extra mile to help you out. Worked for me multiple times.
Join the Porsche Club if you haven't already. Many dealerships offer 10-15% off parts/labor for members. Based on experience the $60 membership fee will pay for itself quickly.
BTW, welcome to the forum. Make a habit of using the search feature before posting. There's a wealth of information here. Not too many questions that haven't been asked or problems that haven't been seen.