OEM or aftermarket performance parts? There's lots if choices. In no particular order...
Mini Mania, Moss Motors, Turner Motorsports, Alta Mini Performance, Madness Motorworks, Out Motoring, Way Motor Works, Bavarian Autosport, Pelican Parts, ECS tuning, just to name a few. Which one is best? It depends on what you are looking for. Prices and service should be similar as it's a competitive market.
Plus, there are some great forums to help you modify and maintain your MINI. Check out Motoring Alliance for for some excellent DIY write-ups, North American Motoring, and MINI2.