Stranded 22-28 awg wire. Search eBay for 26 awg stranded and you will find some. You can also find some on google. There is seller on eBay named Navships or something like that who sells nice 25ft lengths of Silver plated copper wire! stranded 26 awg. The more strands, the more flexible. So a 19 strand wire will be more flexible than a 10 strand.
Some prefer to use Mogami cables, such as the W2944, but I prefer to use single strands and then sleeve and braid them.
Do note, as AWG (gauge) increases, size of wire gets smaller. Most speaker wire is 14-16 awg. But for headphones, 22-28, depending on use is best. For IEMs, prolly wanna use 28, while for a desktop cable, 22 would be better