I always get a pack of mixed o-rings off the bay.They may not be as hard as the originals but you get a lot in a pack that come in useful for less than the price of one original I find.
Currently using B&Q, 3/32" "Plum sure pack" Just shy of 1"diamiter for P17 piston. Seems to be holding up well!!! Air leaked from breach seal ( you can hear when this happens) Stripped and removed valve assembly, cleaned because grease had been sucked in and gummed things up. Pellgunoiled O rings.Put back together and shes back to full power.
I always get a pack of mixed o-rings off the bay.They may not be as hard as the originals but you get a lot in a pack that come in useful for less than the price of one original I find.