AutoPartsByLou.com has them in various lengths and weight capacity. Make sure that you get something with enough push. My guess is something in the 75 pound range? You also need something really strong to bolt the strut to, that can take 150 pounds of force on it 24/7 - 365.
They also sell hardware, to mount the strut to the wood. You might need a metal bracket though. Some aluminum angle from Home Depot might do the trick.
I had to replace some of my basement storage door struts. I used 35 pound rated struts, they seem about right and no longer let the door slam shut, or swing open to quickly.
Fred.
