Unfortunately, in this disposable society there is virtually nothing that is worth rebuilding, or, that you can rebuild any more.
All depends on the innards, if its Hein Warner, etc, its worth doing. If its Chinese, pitch it...
Check with a shop that rebuilds cylinders for heavy equipment.
Take it apart and take the seals to a shop that works on hydraulic cylinders. They usually have books listing seals by size and type and should be able to find a suitable replacement.
Check with a shop that rebuilds cylinders for heavy equipment.