If you look at the fine print on the side of the existing lift cylinders you can plug the part number into a google search. Once you find the manufacturer, you can look at the cylinders in the same size range and pick a replacement 2 sizes bigger. I changed my 80# cylinders out for 120# cylinders and have no problem holding the dressed out bed with a 4" memory foam topper i nthe raised position.
heres the link to the thread with my cylinder info (scroll to the bottom post):
Assorted mods to my surveyor
I got the cylinders for $20 ea off of e-bay once I knew the size I needed.