Tow cables were more common than chains on most tanks. Any multi-strand thread with minimal roughness will work fine. The diameter should be about 1/16" for heavy vehicles like the King Tiger and work down to about 1/64" for light tanks/vehicles.
If you want to go with tow chains, some types of cheap jewelry chain make excellent scale tow chain. You can pick up bulk jewelry chain from many arts and crafts or hobby stores.
Enjoy your modeling...