That would be very kind and I'll be happy to cover p+p or make a UKAPS donation (not sure how to do the latter)I had the same issue, they do not sell them, I ended up have to buy 100 o rings of the correct size, I'm sure I have loads left, I'll have a look in my boxes in the morning and will happily send you a couple if i can find them
Will update on this tomorrow.
£6 with deliveryI think this is what you might be looking for
https://www.eheim-aquarium-parts.co.uk/proddetail/eheim-filter-double-taps-seals-7444190
Part number seems to be 800040 but that set appears to have 9mm, 12mm and 16mm O rings (one of each) which doesn't seem that helpful.