You can use almost any kind of string/line for that purpose. You will find that they sound different, and you can pick one you like. Common strings for TT use are silk bead cord, fishing line, cotton string.
String alternative for Eurolab TT
I have bought a Scheu/Eurolab Premier TT, the only problem of which is that the nearly invisible tiny string Scheu provides tears up every few days. Have other Scheu/Eurolab-owners found alternatives/strings? And what kind of knot do you use to tie the knot - mine open quite often, so far.