You can do this 2 ways :
1. get a GTA look body kit composed of front bumper, rear bumper and side skirts from any tuner, such as Helvetia. Wheels are available from either a breaker or eBay. You won't need the flared wheel arches, so that saves you some money.
2. get the original GTA parts from any AR breaker or dealer. You will need front and rear bumpers, side skirts, and front quarter panels. The rear quarter panels are welded on so I would not touch those, I would rather flare them out a bit around the wheels, so the side skirts would fit just right.
I've seen both done, the later is much more impressive than the replica body kit, yet much more expensive.