I use stuff from electronics suppliers, but it has been getting harder and harder to find. Radio and electronics as a hobby is dying. Now, whenever I am ready to throw out an old piece of electronics, I tear it apart to see if it has any coils or transformers to salvage the wire from. Coils are wound with a varnish as insulation, rather than cloth or plastic insulation (which must be stripped). The varnish also takes paint without priming. Coil wire is also vary fine, which is needed.
You need wire in the 26- 30 gauge sizes for most scales.