Are they welded shut, or do they have a threaded nut at one end, usually the top. If it has the nut at the top, a hydraulic shop should be able to rebuild them. If they are sealed, then look for a manufacturers name somewhere on the tube, and possibly any other numbers that might help to identify the manufacturer and the part number. Last thing is that if you can't find any information on them, then a hydraulic shop might be able to fabricate ones similar to what was originally installed. It won't be inexpensive, but it is a solution.