Just a week ago I had to buy the entire rail kit for a bifold door I have. I got the replacement kit (rail and all the other hardware like that which came with the particular door I have). Home Depot carries them and is where I purchased. Kind of pricey. The kit for the small door I have was $13. The kits were in their hardware section. Various lengths available.
I was directed there by asking in the door/window department since that is where they also carry the complete new door kit.
I must say, though, that your pic of the rail seems to indicate that it is much more substantial in quality than that with which I was dealing.
In my case, the rail, which is just extruded aluminum with a lip, had weakened through use and had a wide place where the door would escape the rail.. All I wanted was the rail, but had to buy the kit. I also checked the local hardware store first and they directed me to H.D. for the extruded aluminum rail such as I needed. -- nohup rm -fr /&
Here is another good site. »www.johnsonhardware.com/. I know they used to supply the hardware for a lot of door manufacturers. Take a look or maybe send them a picture of what your looking for. -- I know you think you understand what you thought I said, but I am not sure you realize what you heard is not what I meant. Nixon