There seems to be a vast choice here:
https://uk.rs-online.com/web/c/cable.../cable-lacing/
Unfortunately they seem to have discontinued the natural fibre types in favour of all plastics materials. The prices are something of a shock, too!
I have the remains of a large roll of R.S. waxed lacing cord (Fibre ?) which I bought in the '60s. Still good and not lost its strength.
Emeritus suggestion (#4 above) seems an excellent way to get a robust, inexpensive substitute that is possibly better than 'proper' cord. I agree with MOFYA Andy that plastic cord tends not to remain in the intended place. Tony.