On the hydros will be a small bar code tag.
Usually there are two or three on them, a couple on the actual units and one more on the chassis of the mower.
Along with the actual bar code will be the makers name and model number.
Using this information you can go to the maker, or a retail parts supplier and buy the parts needed.
Hydrogear and Tuff Torque supply most and they have both the parts books and service manuals available free on line.
There are some on this forum who are really proficient with web searching ( unlike yours truly ) and will generally post links to suppliers or service information almost instantly.
Others are in trade and will post links to pages only available to trade searches however a lot of these expire after a short time.