Yeah, those are fairly easy to strip and you're usually SOL. Subaru might have it listed as the whole system together so it might not be possible to order that line by itself.
What I would do is just use a pair of vice grips and save the trouble of replacing. If you're only aiming to replace the stripped nut you can just cut off the brake line, replace nut (source from McMaster or something), and flare the end of the line. Nothing too complicated for a DIY. Could source something from a wrecked car. Check
http://www.car-part.com/ or a local junkyard.
If you're not lazy like me I would get some new lines and bend it to shape. Looks like pic shows the front brake lines so it's nothing that bad. If it's the driver side front then it's super easy. [ame="https://www.youtube.com/watch?v=bqyoTWFZ5K4"]https://www.youtube.com/watch?v=bqyoTWFZ5K4[/ame]