Tom Morris, MDS Supply (Mel Schuette), RSM Distribution (Eric Rule), McMaster-Carr, New Jersey Wire Rope Co. (10,000' and up), 7-Strand Corp., and doubtless many others sell the stuff. It's available in 1 x 19, 1 x 7, 3 x 7 and several other configurations. 1 x 7 is the stiffest; multi-strand stuff goes around curves more easily, but that shouldn't matter for lines.
Ralph