The 24x3 is definitely obsolete and has not been available anywhere for many years.
I checked the singersewinginfo site and there is a special page explaining all about the 24x3 needles at
http://www.singersewinginfo.co.uk/toys/24x3/
I think it will answer all your questions and it even tells you how to use a subsitute!
I tried it and it worked perfectly on my little Sewhandy 40K.