I (sorta') feel your pain.
I think Cavie's (esp. comment# 2) had a good suggestion ~ "Any good independent mechanic can make one..." but it's not about the costs. Instead, you've unwittingly fallen into the "Do-It-Yourself" project trap. You can't just drive the trike to a shop and say "Fix this please."
I don't think the parts counterperson was helpful ~ it's not good advice to cobble a couple of shorter brake lines together. Instead, you need to approach an independent mechanic, and ask for help. The mechanic should be able to find a long-enough steel brake line, and the experience & tools to bend it and double-flare it.
The hardest part of this whole exercise, IMHO, is to find a talented experienced mechanic that will tackle your brake line do-it-yourself project.