I would recommend just buying the premade lengths, because I've heard that trying to flare the lines can be a pain in the ass, and sometimes doesn't work that well. The only issue with the premade ones is that they'll probably be too long, but you can just bend the lines a little more to take up the space.
I think most auto parts stores sell them by the foot, up to 5 foot lengths. Get more than you need, you can always return the extra ones if you don't use them.
If you end up doing a lot of brake lines, buying the flaring tools will probably be worth it, but for a one time thing, the premade ones will work out, since you don't have to worry about flaring them properly. Just bring the old line connection to the store to make sure you get the right one.
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