Welcome to CPF Fibonacci,
For the Arc AAA o-ring : 9mm x 1.5mm Buna
I prefer and use Poron/silicone construction. Buna is almost as long lasting and much cheaper. Arc will send you a free o-ring and foam kit, if you request by email. I would just send a SASE with my request by snail mail,. imho
Peace,
T&R