The cartridges used in an old Dimension make great spools, except you don't see how much filament is inside.After having put a PLA inside sign on it (http://www.thingiverse.com/derivative:21217), I thought to replace the useless chip instead :)The text is a little too small, still readable but I'll improve it. Otherwise, I may try to adapt or make something to just hold the spool alone (49-52mm dia. for the center).Open a used cartridge. Remove the remaining filament (empty don't mean there is nothing left in the case of these cartridges, usually 20-50m of filament). Wind the filament of your choice on the spool (not sure if it works with 3mm filament). Put back Feed your hungry bot :)For a real look inside the eeprom, read this article from haveblue : http://haveblue.org/?p=938