Hello bobby2!
Flex wire eh.
It is more than likely that you are talking about the lead wire that connects to the magnet wire of the coils.
Common types used in electric motors are HYPOLON "class B-105 degree C", EPDM "classF-150 degree C", and SILICONE BRAID "class H-200-220 degree C".
All are rated 600volt maximum, may be either soldered with common solder or a 5% silfoss braze may be used. The term "flexible" is because the gauge size is made up from very fine wire giving it much more flex than standard household or wire which would be used in conduit runs.
Easily obtainable from electric motor repair shops in all sizes.
If you wish to order from a motor rewind shop or their suppliers, do a google under E.A.S.A. E.A.S.A stands for electric apparatus service association which all good motor shops are members of world wide. You can find at the web site the complete listing of all the motor shops complete with their phone numbers and e-mail addresses if available. Also, if you look under "associate members" you will find all the major suppliers of proper rewind materials that supply the motor rewind shops and motor manufacturers.
hope this helps, have fun!
zubbly