I buy mine at any local craft store. I also always buy wool yarn for added durability.
I wrap about 15 times around my hand, leaving long tails on each end. I slide the looped bundle off my hand, split the string, insert.
Now you have the loops on both sides of the string. I then use the tails to tie series of knots on both sides the the yarn bundles for added security.
String the bow up, shoot a few arrows, then snip the loops with a pair of scissors.
One bundle of yarn will do 30 to 50 strings for only $8.00