I've seen some posts around here from people asking or talking about this, so I just thought I'd share some info, for anybody interested; even though you can't really see it installed unless you look up under the tank, and even then you don't see much...
Anyway, surfing for bike parts just now, I found two manufacturers of it. Parts Unlimited makes the blue, transparent line, sold at alot of places, including Bike Bandit. The hitch is, I only found it sold in rolls as small as 25ft, and up, and its starts around $40 and goes up.
(I wasn't looking for fuel line so I didn't bother searching for better sizes or pricing)
But then, (better news), I also found Helix Racing makes it as well, and in 4 or 5 different transparent colors, and while they also will sell large rolls, I found it for sale packaged in 3 ft sections also, for about $6. And available at a local dealer.
They both come in sizes that fit these bikes, 5/16 ID, as well as others if you have other projects.
So there it is, if you're interested-Parts Unlimited in blue transparent in bulk rolls, or Helix Racing in a variety of transparent colors, sold in bulk rolls, or usable sections. They are both fuel/oil specific, and say they will not "harden".
There's probably others, and better deals, but I came across those two, have seen it talked about here, and just thought I'd pass it along.