Right here on Core77: Desktop-Sized Machine Allows You to Create Your Own 3D Printing Filament - Core77
The spool inside the Next 1.0 filament extruder actually is the very SD 200 K made by Hafner.
http://www.hafner-spools.com
Somehow it has wound up as the de facto standard for FDM printing. Some of the printers do also take the larger SD300K or you can put the SD350K on an external spool rack, since all of the spools have the same central bore.
Originally this set of spools were designed for the needs of wire welding applications. Thus combining relatively low cost with strength and precision. But at the moment I am actively looking into better understanding the needs of the 3D FDM printing market and how to fine tune or adapt some spools to that.
mo-i