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Or pop into halfords and get a 500 o-ring set for £4.99 and avoid paying subaru 4 times that for a set of 8. The halfords set contains the 2 sizes you need.
As I recall the injectors have 2 o-rings each, and then a little flat o-ring. The halfods kit has the right sizes for the 2 orings, but not the flat rubber ring, just re-used that when I did mine.