The rear wheel arch ones are available from any Official Porsche Centre. They are known as Stone Guard Foils and are about £40 per side inc VAT. If you contact them with your chassis number they will ensure they get the correct ones for your car. Be cautious about using any from some of the independant suppliers as I bought some and they were not the same size as the originals, sent them back and bought Porsche ones no difference in price !! The originals have Scotch 3M branding on the backing.
They come in various colours depending on the body colour of your car. White and Silver cars are colour coded whilst those for dark colours are clear. Think Porsche did this as the clear plastic turns brownish in colour when subjected to UV's from the sun over time.
For the other areas of your car, at Ragley Hall there was a company called Ventureshield who seemed to make vinyl protection for all parts of the car, if yours is a light colour I would ask them about the ageing problem as they appeared to be using the clear plastic film.
Removing the old ones is not too difficult, I found that using a hair drier on hot setting gets the thing started, you can be quite brutal pulling them off, using the hair drier occasionally if things seem a too well bonded on. I found that if you use the hair drier sparingly then more of the glue stayed on the old foil than the paint. Any glue residue was removed with white spirit followed by panel wipe to clean the surface before applying the foil. Ensure there are no hairs or dust on the surface prior to starting to fit the foil.
Fitting is not easy and I messed up one when fitting and had to get another. The trick is:- mix, in a plant spray bottle, about 3 drops of Fairy liquid with water and shake to mix. Wear a pair of clean rubber gloves to avoid any finger prints on the glue surface, spray the cars surface with copious amounts of the liquid, and with assistence from a second person peal off the backing complete. Lay the foil on the car, the liquid allows the foil to be adjusted into position, then using a hard edge piece of plastic like the mixing pallete that you get with P38 type filler squeeze out the liquid from underneath by pulling across the surface to the edge. I found that the odd bubbles that seem inevitable, if pierced with a pin, squashed down with your finger and then allow sun to heat it up for a few hours cleared them.
Sorry about the long reply but thought it might be useful to pass on my experience of this job.
Good luck, Cheers,
Andy B