If it is really "bicycle" stuff (i.e. cable inside a sheath) try your
local "hi end" (you know, the place with all the guys hanging out in
their skin tight shorts) bike shop. The often but the cable and
housing in bulk quantities.

Otherwise, you might just have standard stainless steel cable. get
that at Wicks, Aircraft Spruce, etc. If you need a fitting swagged
on the end, your local A&P may be able to to that or (i think)
Aircraft Spruce may do that.