You can try local machine, bearings or hydrulics shops.
They might have a tray of variuose size O rings.
I would think a smaller ID would work since you're just going to stretch it over the needle to make a seal.
You're not going to use it on a flange/face with a recess for the o ring to make a seal..per say.

Sometimes I slip fuel tubbing over the needle assembly, under the click thing of ma loo.