Spares are readily available from Germany: Versandhaus Schneider I believe. Complete kit of piston/seal and spring very reasonable price. Remember to get the 'export' spring. Delivered to your address as well.
If you try / succeed in getting a new piston (i.e. complete piston not just the seal) from Armex I think it has to go to an RFD. So you may have to order from your local shop.
Cheers, Phil