As far as fly tiers are concerned there are two kinds of copper wire from electrical/electronic sources. The insulated wire that isn't coated, and the wire from coils that is. The original Pheasant Tail Nymph was tied with coated copper wire. There are patterns that call for old tarnished wire

As I use a lot of colour coated copper wires I do not buy the ones spooled for fly tying any more. Beading supplies are a good source. Locally a spool of UTC wire will cost me about 2:20 UKP for about 10 yards. Similar wire from beading supplies costs me 1 UKP for about 100 yards. Even better is that the fly tying wire usually comes in sizes Small, Medium, Large, Brassie, as a sizing system this doesn't really mean much to me. Your idea of small may be my idea of medium. The beading wire I buy has a stated size. Most often I get 0.3 mm and 0.4 mm. Now that's a size that means something. This wire is so cheap it isn't worth the effort to search out old electrical coils any more.

One of the best sources I've found is here. This is UK based but I'm sure that there will be a company selling similar your side of the big pond.

Cheers,
C.