I'd try a store that supplies eithert medical supplies to chemistry
equipment. If there are none in your area, go to the local highschool
and talk to the Chem Lab instructor about where to find this item. Or
you might try to run a Google and see what comes up.
Or try a radio control modelling shop, they sell 'bowden cable' consisting
of a twisted wire core with a plastic sheath, the stuff I use is just under
3mm diameter but there may be smaller alternatives.
For shortish lengths find some plasic coated wire of the right diameter, cut
round this with a knife and slide the plastic off the wire.
Regards
Mike