(Yes, I am aware that this thread is really old, but it was the only one I could find that adressed the hoses used in the Legend system.)
I just had the same experience with my install. The line held it's pressure for about 75 miles, then blew. The Legend air hose looked as though it was pinched somewhere, so I'll chock that up to user (installer/me) error on my part.
I too searched around and couldn't find a suitable replacement. I then remembered that the hose used for the stock "adjustable" shocks on the FLHTCU uses a similar size hose. I stopped by Harley and picked up 10 feet of that hose and installed it. System worked great up until the tube got too hot and popped like a balloon. I heard it pop and then heard the hiss. That was while riding 70+ on the interstate. Tried to temp fix with duct tape which held for about a mile. Ended up limping back home with my new "rigid frame" MC.

I'm now trying to get a 60' piece of the hose from Legend. I see carrying a spare with me as good insurance. It doesn't take up any room, and it's fairly easy to swap out with minimal tools.
As far as the comment on the diameter, I did find some tubing from Auto Meter which had the same outside diameter but a larger inner diameter (which didn't lock into the fittings). I'm pretty sure that the smallest inner diameter is used in this application to reduce the volume of air required to pressurize the system. I was thinking about using metal tubing, but the inner diameter would be large and that poor little compressor would have to pressurize that line and the shocks. I expect it would shorten the life of the compressor.
As a new cold wave rolled in yesterday, I hope to be back up when the next 40+ day arrives (Thursday). The riding season here is short enough and these kind of delays are frustrating. At least I've still got my Dyna to ride...
If anyone does know of another vendor that sells a suitable substitute for the hose from Legend, could you post a link?