You need an APC solenoid and a few pipes to fit it- an easy job as the wiring harness is already there. The APC solenoids sometimes come up on eBay, or you might find one at a scrappy. The hose is 8mm ID and silicon can be used.
You will need to get your existing ECU reprogrammed. There are several companies that offer this, and they normally offer the APC solenoid and hosing. They may also carry "spare" ECUs in stock and operate a core charge refund system, so that you don't have to be without an ECU for your car whilst it is being reprogrammed.
Abbott Racing
Maptun
Speedparts from
Elkparts