Take the both off and to a local hydraulic hose shop. They have LOTS of o-rings - get Viton o-rings. If they don't have the exact size, buy a couple too small and too large. O-rings aren't critical in size, they don't have to be a perfect match, just pretty close.

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There is a very fine line between "hobby" and "mental illness."
FrankenBenz: '87 560SEC with an '02 cammed LS6 and custom 4L60E.
Bling: '87 560SEC, ported/polished, lowered
Lisa: '87 560SEL, lowered