Lately, I've been using the thin hemp cord from wally world (not the thick stuff). You can find it in the craft section with the jewelry making stuff. It says 20lb test and that's pretty accurate. I've done my own tests and I found that the average is about 18lb breaking strength.
Another option from the dept. stores is to use multiple strands of thread. The typical quilting thread (63% cotton & 37% polyester) tests at about 1.8lb per strand. This is about the mid range for thread. Note than many threads will stretch quite a bit, like anything nylon, so read the spool. You don't want a lot of stretch.
Hemp doesn't stretch much and is a good, cheap alternative to flax or linen. Some purists may think anything from the dept. stores is trash (most of it is, I agree) but if you test the material beforehand you can get good results.