A good one is one that will contain the pressure you plan to run it at and has a bore large enough to give you the force you want (you will need to run the numbers) with the stroke that you need for your weapon design.

In terms of locating one I'm assuming we are going second hand? In which case, ebay, gumtree, local scrap yards, farm auctions, dealers in farm machinery etc would be your best bet. You could go for a custom solution but of course expense comes with that unless you can machine it up yourself.