I buy from McMaster Carr. I used to buy big spools from Strandflex, the ultimate supplier of Sig, etc.The current crop seems to be some offshore product, possible made to metric standards and sold as .012', .015", .018",etc, but actually just a bit off. Even Brodak's product seems to be just a bit off nominal size.
If you are going to use stranded lines stick with the seven strand lines. They are much more durable. As Mark said, the 19's are very fragile. Each strand on the 19's are less than .001". They will fray much quicker than the sevens, at least that has been our experience with them on our concrete circles. Doug
Agreed, I think the 19-strand are more compliant as well.