To get the last Bottle, you have to find all 10 Big Poes in Hyrule Field as an Adult. Ride Epona and when a Poe appears, hit it with 2 Arrows to destroy it, then get off Epona, approach the yellow flame and put it in a Bottle. Take the Big Poes to the guy in that room before you enter Hyrule Castle's Market; here's the locations of the 10 Big Poes...
1) Near the stream to the back left of Hyrule Castle.
2) Just outside Hyrule Castle by the sign that leads you to Lon Lon Ranch.
3) Under the ledge near the entrance to Kakariko Village.
4) By the tree right outside Lon Lon Ranch.
5) Where the stone wall turns.
6) At the tree below the ledge near Gerudo Valley.
7) Next to the wall between Gerudo Valley and Lake Hylia.
8) By the boulder between Lon Lon Ranch and the Lost Woods.
9) Where the patch of grass is by the trees between Lake Hylia and the Lost Woods.
10) Amongst the trees near where #9 was.