I usually pay $1 to $2 on E-Bay, and buy a few spares but then the wait may be shipping from China which can be sporadic, especially now.

I generally choose stainless cable, but galvanized can be used in a pinch.

One can often recycle rear derailleur (or brake) cables to be used in the front, but you'll still need new cables.

Oh, were you talking about cable housing? The department store kits will generally come with unlined housing. Generally very little choice in colors. Black?