It's been along time since I had mine out to clean (ISCV obviously), but I don't recall anything significant about the o-ring. Any off-shelf o-ring kit would be fine.
Alot of the Ebay ones are Legacy/Forester and/or different partnumbers when I last looked, so make sure you pick the right Part Number 22650AA240
This seems to imply it fits :
http://www.ebay.co.uk/itm/Subaru-Imp...item4ae6089adc