length (compressed & extended) AND weight lift. Each lift cylinder is rated by how much weight it will support. MOST automotive ones have a higher weight rating (think hood, rear hatch, etc). West Marine would be one choice, another would be Kendor Marine. I bought two 80# lift supports from them when I built my casting deck. That was trial & error for me not having any idea how much support I'd need to support a 3x4ft deck lid with a seat & pedestal installed on it. They gladly let me return them until I found the right support for my application. They literally have boxes and boxes of various length x weight models as well as all sorts of different mounting brackets.
If you can read the numbers off your existing ones, it's easy to get replacements, if not then you'll need extended and compressed length measurements, then guess at weight ratings by how much weight it needs to support.