Need recommendations on 15ft speaker cables

Hi, 

I find myself ina situation where I need around 15ft cable pair. Currently I have blue jean Rca cables running. I have upgraded all the other cables and this is the final frontier. Online it is pretty tricky to find used cables of this length.. I see transparent reference cables mostly, which I learned are custom tuned for specific gear.. 

Looking for suggestions from the community on options that will provide SQ upgrade in my situation through speaker cable change..