The buckle is available from porscheshop uk. I purchased mine there. No question of fit, works perfectly with my deactivation bar. This was the only source for this product I have found. Alternatively, you can purchased a used Porsche car seat that has the buckle attached and remove it for use with your car seat.
My 1998 986 has the following setup: 2014 Porsche Booster seat (no buckle, bought new from dealer who matched on-line pricing), deactivation bar (installed by me, verified and activated by dealer), and the Porscheshop.uk buckle. All works like a charm. The setup was expensive (seat, deactivation bar, activation of the deactivation bar (PIWIS dealer), and the Porscheshop.uk buckle (about $25+ shipping to USA).
Seats Baby & Child