Fantasy/Fiction, Elves build a physical border separating them from Humans...

I read this book about 3 or 4 years ago when I was in Highschool. I have no idea who the author might be, or can even come close to what the Title could possibly resemble.

The setting takes place in roughly the same kind of "Era" our world is today, but from what I can remember is was more Run Down.

The Elf society built a border separating them from Humans, and no human is able to cross it.
The main character is a boy who has an "elf/magical" ability where he is able to find anything (a half-elf asks him to find his younger sister, and he is able to "sense" where she is) thus it is his Job and what he is known for is his ability find/locate anything.
He also befriends an Elf who has a "human" ability, she's basically a genius mechanic, I think she used to be some sort of Elf Nobility that chose to live on the Human side of the border.
At the end there is a virus that ends up infecting Elf-kind on the Human side of the border and the boy's Elf-friend dies. The boy ends up inheriting everything from his friend. His Elf-friend's brother eventually shows up and he has to tell him that his sister died from the virus.
The Elf-friend had kept only one thing from her homeland, and that was a bottle of some sort of drink.
Any information would be appreciated, thanks!

That sounds very much like Finder by Emma Bull, a novel set in the Borderland series.

That's it!! Thank you so much!