My mom made one for us and she used a plastic, fishing line. It did break after a couple of months. My toddler was more interested in wearing it herself than playing with it during nursing, so it took some real abuse.
I think fishing line would work. You just need to get a strong enough "test". If you go to the WalMart fishing tackle department you should be able to find a 40 or 60 lb test and that ought to work! GL!
Hemp and leather cord is also very strong and often used for nursing necklaces. Here's a few more tips:
1. be sure you make it long enough so babe can move it around and not choke you!
2. tie a knot every few beads so that if it does break you won't have so many beads falling all over
3. If you use hemp or leather cord you will have to buy beads with a larger hole-easy to forget this until you sit down and say-oops
4. some of the glass beads can break if knocked against something hard so just be aware
5. you can make some really cute ones with inexpensive wood beads from wal-mart or Micheals.
6. be sure to tie that knot as tight as possible when you're done, then tug on it hard to see if it will give