I made up a list of the best battles (in my opinion) in 1st quest Adventure Map to farm all of the Giant Bosses and enemy heroes. You will not always get a material though, a lot of the times you get weapons. But with + materials weapon affix, and a lot of farming you can get the materials you need eventually.
When I say (for example) B11 x2 I mean that square as that boss 2x in it. And x3 if the boss is there 3 times.
A1 is top left and H16 is bottom right of the map.
Ok here is the list.
Agnarok - B11x2, D16
Dodongo - B6, C7 x2, F4 x2, F14 x2, G7 x2, H13 x3
Ganon - B6
Ghoma -B6, B12 x2, D16, G5 x2
Imprisoned - D16, H3 x2
Mandala - B11, C10 x2 E2 x2
Agitha - A15, C4 x3, C8 x2, F6 x2
Darunia - B13, E13
Fi - H6
Ganondorf - A5, A15
Ghirahim - H6
Impa - B2, E13
Lana - A13, B10 x2
Link - A5
Midna - B10, C4, F6
Ruto - A13, A15, B13, D10 x2, E13
Sheik - B13, C11
Volga - D12, E16
Wizzro - D12, E16
Zant - B2
Zelda - A5, B2
I don't have a good map square for Cia so if anyone wants to share that info with me, I'll add it to the list.
[edit] I have found out, that Legend mode stage where Cia splits like 4x, you get a drop from each clone. So that could be a good way to farm her materials. Not sure if the difficulty affects the drop rate silver vs gold materials though.