Take that o-ring to a local hydraulic shop and hope that they have that size in stock.
This is assuming, of course, that the main seal in this case is an o-ring and not an u-cup. You have to measure the inside diameter of the o-ring (best done by measuring the groove on the ram) and the thickness, or cross section, of the o-ring. They only want to repair expensive equipment so it is up to you to take the jack apart and remove the main seal at the end of the ram. Most hydraulic shops don't want to repair low priced imports, because it is not cost effective for them, so they tell the owner that the jack is not repairable. Some may have odd sized seals that are not carried in stock by local hydraulic shops because the number of o-rings available and u- cups is very large and it would be very expensive to carry all sizes.
