Fasttech does not sell these o-rings on their own for these tanks. They are on the Kangertech website and a few other shops that are US or UK based, separately or you can buy the after market replacement pieces for those tanks here and steal the o-rings from there.
SKU 2034311 , etc (drop down), but alas they are a bit expensive.
A wise man once said "again I see Jaco ... I am not a pirate ~ptitfour"
Vurt da Furk?