There may be a specific forum thats dedicated to custom cars based around your area, also worth looking at places that do american customs as they will have paint facilities.
To be honest I think £500 is a bit on the low side, for skirts, spoiler and wing, esp if they need to be painted and prepped. Also lots of places wont give you a final price till they have seen and test fitted the kit (or at least they will have some type of caviat!)