Not generally available these days, so it is old dealer stock, or make your own.

I tend to use microbore central heating pipe as it is copper and does not rust, it is easily formed and easily fitted. I solder an olive to each end to give the flexible hoses something to grab on to.

If you use copper they will be fitted for life, no more problems but will need isolating from contact with steel, i use the plastic clips screwed to the chassis.