When I needed very long springs for my GTRV pop-top, I was able to get them from Gemini Gas Springs. They were very helpful:
http://www.geminigassprings.com
IIRC, it's telephone-orders only, but they have a myriad of options for both the spring type and the end-fittings, so you'll want their help with ordering anyhow. Glancing quickly at their catalog I see 40" long units in both their 14/28mm line and their 20/40mm line, depending on how much force you need.
As a bonus, they were somewhat cheaper than the 1st set I tried to order from Faucher Gas Springs (the OEM for the GTRV part I was trying to replace - but their "replacement" isn't a bolt-on match.)
If you're replacing an existing spring, try to have the part number or specs handy so you can figure out an approximate replacement (or upgrade, in my case, since I went about 50% firmer).