I once bought a "ready made" AutoCAD symbol library, I think it was said to contain 10 000 symbols. It was hard to navigate and I only used like a dozen of symbols from it, complete waste of money.
I suggest buying the ISO 1219 standard so you have the basic measurements of the symbols and then make your own symbol library in AutoCAD. Start by creating the really small basic parts, like flow arrows and symbol squares. Then you group them together to create valve symbols. Put the base point in a smart place. Be consistent with what you do. Once you built up a library it is quite fast to expand it.
I started using the part numbers in the ISO standard at first but dropped that pretty quickly, it was just a mess and didn't help me at all. Now I just go with the autogenerated block numbers which is fine, I don't use them for anything anyway.
When it comes to stuff like variable pumps, variable motors, I use the manufacturers symbol from the datasheet instead of the generic symbols from the ISO 1219. It takes a little bit of work to model up a variable pump but I'm guessing the amount of different pumps you will use is limited. Also, most of the components in a pump regulator for instance, you can pick from your own library. Same with LS-valves, I recreate the manufacturers symbol from the datasheet, making a building set out of it. Spool is one block, actuator is one block (probably same as ISO) then i build up a section like that. Then stack sections to complete valves.
