pettitracing, rotaryextreme, malloy mazda, a-spectuning, etc. there are a lot out there. decide on what part you want and shop around for the best deal. if you can find a rotary specialist locally that can get you great deals on parts, that would be the best option.
if you are going to start racing your car, look into Mazdaspeed Motorsports Development. They have a program to help racers by getting them discounted parts.