Carp,What do you want to do with the hydraulics? Pressure & cylinder dia determine what you can move. The higher the pressure & bigger the bore,more power the cylinder can deliver.There's a few places to git the parts from.But none of it is cheap
,& if it is don't bother purchasing it.
The cylinders I use are 7mm up to 16 mm bore. Tell me what you are out to make,& I'll be glad to give you the best route that I know of,to get you going.
FGD