It appears your Pantera is equipped with 15x10 rear Campy wheels, and I'll assume it has 15x8 Campy wheels in the front. The European GTS was originally equipped with 225/50R15 front tires mounted on 15x8 wheels and 285/50R15 rear tires mounted on 15x10 wheels. I assume those are the tire sizes you're interested in. Tires in the 285/50R15 rear size just aren't available these days. Everybody opts instead for 295/50R15 tires for the rear, there is reasonable availability of passenger car tires in that size, but only with lower speed ratings.
A side note: for those interested in upgrading to this tire/wheel combination, 15"x10" forged aluminum Campy clone wheels for the rear of the Pantera are available from a seller on eBay by the name of MisterFiat. They are manufactured by Roin Technology of Italy.
Good quality 295/50R15 performance street tires appropriate for a sports car are available, but locating and acquiring them can be challenging and expensive. For instance Longstone Classic Tyres in the UK sells an Avon CR6ZZ in 275/55VR15 or 295/50VR15, but they don't offer a 225/50VR15 for the front in the same make and model. They do have other makes and models (Michelin, Pirelli) in that size with V or Z speed ratings. Its never ideal to mix and match makes or even models, but when coping with acquiring tires for the oem 15" wheels its an unfortunate necessity. The tires at Longstone are high speed rated and available.
The difficulty and expense of acquiring performance tires designed for 15" rear wheels has lead some owners to utilize street legal competition tires. Other owners have chosen to use tires designed for light trucks (rated LT) in the rear. These are unusual choices for sports car tires, I understand why owners make those choices but I don't wholeheartedly agree with them. Street legal competition tires are not the safest choice, and tires with sidewall structures and rubber compounds designed for light trucks are not a good match for an Italian sports car either. But those tires are easier to acquire and/or less expensive.