I've used the Jute twine found at the local big box home store. I made a double run from the soil to the top of the post. I even put it in a drill and twisted to maximize strength. Still... After being wet all season and having the weight of the plant, the lines broke and I had to re-tie 3 times.
I did find coir twine in 425' rolls of 6mm diameter. Pretty much what the pros use and it was pretty inexpensive.
Check it out and see what you think. I'm using it for the drops and I like it a lot.