My dad always had a Hein Werner jack. It was left from his garage business in the 60s. It always worked great. I think he's still using it.
I am not a hydraulic guy. I usually have success when working on these sorts of things, but it is not my trade. I am sure there are others who would have success where I did not. I am just sharing what I have run across.
I found a used one that the PO thought was junk. I thought I could rebuild it. Things were not good inside. I rebuilt it more than once. I think I honed it out at some point. After awhile it would creep down & needed fluid periodically. I finally gave up & bought a new noname brand. Not the cool factor of that old one, but it has served me well. Far less frustration. I should have given up on the HW years earlier.
It's funny, I threw the HW away at work. A buddy pulled it from the trash. I told him it was junk, but he had to have it & was sure he could rebuild it. I never heard how that worked out. I'd still grab another if I had a chance, but I'd be more careful about the condition. One in good shape like my dad's would be nice. If it needed more than rubber parts it was beyond my skills or the work required to make it good.
It has been years, but I'm pretty sure I got the parts from Cal Tool. At the time I worked at a hardware store, & we ordered from them all the time. It looks like they have been swallowed up by
MEDCO, & they appear to be nationwide & wholesale only. Following the links to Hein Werner, it looks like they are part of
Shinn Fu of America now. Following their links to service, it looks like
this page should take you to a local service center, who would probably be able to provide parts if you don't find them thru the sources listed in earlier posts. Or, if that doesn't work for some reason, you might try thru MEDCO to find a local retailer who can order the parts.
As mentioned above, does it need a rebuild? After seeing my dad's work so well for so long I might hesitate to tear into one that was working correctly.