Cling Vinyl and Chromeboys
Cling Vinyl / Window cling etc. (comes in various names) - you can get this stuff from people who make signage for window displays. It's the same stuff they make vinyl letters and graphics out of to put in shop windows. The key thing to remember is to ensure it's clear VINYL and is the static cling type - i.e. it DOESN'T use *any kind of adhesive to stick!
I can't imagine how a 'guitar specific' one could be any better. Someone feel free to correct me if they know otherwise, but I guess cling vinyl is cling vinyl. The advantage, I would guess, of buying the vinyl yourself and then cutting it to shape is that you can shape it to go exactly where YOU want it to.
You can get it in various thicknesses and various lengths. It isn't too expensive either. I just got a 5 metre roll (30 cm wide) for £20. Should last for ages!!
I live in the UK and the people I bought from were kind enough to send me a sample first and then, when I ordered, delivered the next day. If anyone wants to know who they are, let me know!