This is a line that operates under pressure, and has special fittings. Autoparts store stuff won't work.
You can have a line made up at any local shop that makes up hydraulic lines for heavy equipment. You should be able to find one in the phone book.
I'd take the line with you...and make sure they can duplicate the 'banjo' fittings on the ends, or maybe use the ones on the old line.
You'll need to figure out how much more length you want.
You might as well go with braided stainless line while you're at it.
There was one aftermarket company of braided lines for bikes that had special banjo fittings with a universal coupler on the hose end, and you could order a variable lenght mating hose to come up with the length and connector configuration you wanted. I forget who that was, though...
If you find a shop that makes up the lines inexpensively, might as well get the rest of them done. The OEM rubber ones do degrade after 2-3 years or so.