Bill brings up a good point , if your jack cylinder doesn't have a bent or rusted shaft and the bore is clean most good hydraulic shops can rebuild them. The seals are usually regular hydraulic compound seals which a hydraulic shop can match or substitute with a superior seal Some of the cylinders have to have the upper cap cut off in a lathe and rewelded when done but most can be accessed from the bottom .
You may want to contact your local hydralic shop either way the cylinder will have to be removed for service.
Advice if your system has HWH cylinders you may be able to get a number off the cylinder and get a hold of an HWH dealer for pricing on a rebuilt cylinder for comparison you might be surprised.
Eric