If I were you, I would save the money and buy some 3M heavy duty tape and do the installation of both the splitters and spoiler yourself. Easy to do, there are DYI for them here as well for guidance. Just need a heat gun to remove the back spoiler that is on there now.
As for the gasket, there's a few shops which I've heard good things about: Turners, Ace Performance, Kaizen, Bren Tuning, Bavarian.
Shop around for best quotes.