For electrical insulation? Rubber of course would only be made stable with invention of the vulcanizing process in the 19th century. Cloth tubing of fine thread would have been possible to weave, of course- something that was used well into the 20th c. But for a rubbery, plastic substance the closest would have been a varnish. An oil varnish could have been brewed up, using a drying oil like linseed alone, or mixed with gums like damar for more durability . Or a spirit varnish, using shellac and other gums dissolved in alcohol.

If you search under "magnet wire", you will see that varnish can actually work pretty well as electrical insulation.