With all of the discussion lately about double-flaring tools, I just assumed that all you could buy was straight lines that you had to flare youself. If these pre-flared lines are fairly common than that explains why they're so cheap on eBay.
You can buy the stuff in bulk, but everyone I know buys the straight lengths with flares and nuts both ends, then cuts them off :-)
A good store (nearly none of them these days :-( will have lots of lengths; it's conceivable to replumb your entire brake system with standard lengths.
You need a good bender though, one kink in a bent line and it's trashed. You can bend gentle curves by hand but go suuuper sloooow.
When I did the rear axle on my hornet, I used standard lengths; one was about 3" too long so I took up the slack in a slightly generous bend needed anyways to clear the U-bolts. A simple, easy and safe trick.