I started buying stranded wire from Amazon, cutting the sheathing and separating out the colors.  I found this to be less expensive than buying separate colors or even color sets often.  These spools work great for re-spooling.

These fit together like a glove so I recommend giving the inner shaft a tiny sanding before assembly, like one or two spins.  If need be you can tap them together with a small hammer. lightly tapping the center.  It's a great design just wanted thicker walls and gave better information as to gauge and length the spool will hold.