You can upgrade the casing on a 2 button keyhead with a $129 model that is from the Carrera GT. I see them on eBay now and then. You take out the key part and the electronics and put it in this new casing that's very cool indeed:
http://e-partssales.com/Merchant2/merchant.mvc?Screen=PROD&Store_Code=P&Product_Code=gthead&Category_Code=cgt
The other option would be to buy a new key head and key blank from Sunset and have your local dealer program the key head and cut the blank for you.
The key head and key blank will set you back about $175. Then, the dealer will probably hit you up for an hour minimum to cut the key and program it. You must leave the car, a working key, and the new key head and blank with them to do the programming.
There's just no cheap way to get more keys or replacement keys for our cars. Sunset's about the cheapest place to get the parts though.
My local dealer quoted me well over $400 for a spare remote key
PS - DO NOT BUY USED KEYS OFF EBAY. THEY ARE WORTHLESS. (Unless they have the original barcoded tag for the key AND it's the model number designed for your year model Boxster, it's a no-go).