We get it at Whatcom Farmer's Co-op- or Cenex. It's a plastic twine similar to baling twine but not as thick. It doesn't stretch like sisal twine does.
You can order it from A.E. Leonard a supply outfit that's on the net-- but I just Googled them & their site is down tonight. It's more expensive there than from Cenex-- how much do you need? If you can't find it in OR, maybe we could get you some & ship it to you by UPS or something. I think it's about $9.00 or so a roll at Cenex- sometimes less if on sale. Walt reuses it from one year to the next & it seems to be good for a two or three years if you don't get it too tangled when you remove it.
The posts are 6' med. weight metal fencing posts- about one foot is thumped into the ground. The first cross piece is 4" above the ground & from there on they are 8" apart up the post making 5 cross pieces on each post. The cross pieces are about 6" long & the holes that the twine goes through are about 1/2" from either end of the posts.
Does this cover what you need to know??

Thanks plantlady, that's exactly the information I was looking for. There are a number of Christmas tree growers in our region, so I'll keep looking for the twine. We're new at growing dahlias, but have put in a bed with a couple of long rows, and your system looks like it would be a great way to support them.