in NJ
I just bought this 4 pack off ebay: https://www.ebay.com/itm/GSXR1300-HAYABUSA-EXHAUST-PAIR-VALVE-BLOCK-OFF-FREE-SHIPPING-/302578279700?vxp=mtr
I only needed 2 so I'll sell you the other 2 for $10 shipped to your place. I will also include a little sheet of felpro gasket maker since the hole in the head is oval and these have a round o-ring. If you're careful you can reuse the original gaskets from the pair system, but one of mine broke so I had to make a new gasket. They look pretty nice against the grey engine, I'd post a pick if I were home. PM me if your interested.
Ken
We liked to use the PAIR system to evacuate the crankcase on the 1st Gen Busas and the liter bikes (to slightly reduce engine pumping losses). You could gain 1-2 hp on those bikes but I don't know if there would be any significant gain on the GS500s. Probably better to just lose the weight of the PAIR on these smaller bikes IMO.