JimK wrote:

Anyone know what I've seen? boats? wire track lighting?

I suspect what you are talking about are rope grips or bulldog grips. In
marine environments these are mostly stainless steel, although heavy
galvanised are also use on older boats. Quite cheap at most chandlers.

I'd suggest that you also consider getting turnbuckle ends for the rope
which will eliminate some, if not all, of the sag.

http://www.s3i.co.uk/ropegrip.php