This ebay auction for a Stitch Mistress has a picture of the instruction sheet
http://www.ebay.com/itm/STITCH-MISTR...UAAOSw03lY7912 If you zoom in at the bottom it says that it takes standard chain stitch needles that should be available at a sewing machine agency. Not much help.
This page
http://www.singersewinginfo.co.uk/toys/tsmneedles/ lists a Genero Stitch Mistress that takes 15 x 1 needles. Again no help. If you go down to the bottom of that page there is a chart of needles size with length which might help.