Should be a hydraulic repair shop somewhere near you, usually disguised as machine shops with "engineering" in the name, or xxxx hydraulics etc
You can measure and order the parts, but usually I get my cylinders back within 48 hours and they normally guarantee them for awhile, though most of my stuff is very abused when I get em so a lot of the rods are bent, and the tubes are gouged... A wood splitter should not have those sorts of problems lol. That said, I pretty much just ignore em and say repack as is... so far I haven't had an issue with any that haven't been guaranteed, and the only issue with one that was rebuilt is because I broke it.
If you do decide to repack yourself, soak the big internal seals in very hot water (not boiling) so they soften up and become a little easier to install... still not what I would call easy, but certainly easier, remember to let them cool off completely and pre lube before reassembling rod and cylinder. And you will need a very big wrench and a very fat man to break the rod nut free, gland end should come fairly easy, just check for a set screw before attempting.