Any good motor factor should be able to supply you with a blank sticky base and letters/numbers for you to add yourself. This has the added advantage over the small motorbike-style plate in being even lighter, thereby reducing understeer and improving the power-to-weight ratio.
As to their legality, I've been stopped a couple of times in cars with stick-on plates (Caterham and Midget), and have had no aggro. Mind you, the policeman didn't notice that the exhaust of the Midget had fallen off and was lying in the boot of the car.
Cheers
Aidan
Aidan Parte
aidan.parte@virgin.net