OK, The replacement fobs are expensive but, save yourself a great deal of time and potential heartache with trying to source a replacement old unit on the cheap.
The fobs are a well publicised source of instant failure and just about all the stories you read about how to get them working again don't actually work.
I would say, Bite the bullet and just get a new one and enjoy the facilities of remote locking and unlocking without worrying about when the fob will die again, they average around 7 yrs of life before they die, so £350 over 7 yrs is 7p per day.
Trying the online repairers and paying for recoding to the car with an old fob - which will fail prematurely will end up costing you much more in the long term.
Are the fobs overpriced - yes, but that's what Porsche does very well - and lets face it, you don't have much choice