Recommendations for flexible speaker cable

Nordost Flatline.

I'm using the Quicksilver Audio cables (www.quicksilveraudio.com), which come only in 4' lengths. Don't know if that's long enough for you. They are highly revealing, pure silver, thin and fairly flexible. Even on my horn speakers, they are not bright sounding. They are $340 for the pair, and I've yet to find a pair under $600 that is as honest and transparent.
Good luck,
Howard

Kimber is very flexible and should fit your budget.

www.rivercables.com flat,flexible,reasonably priced

Supra Ply 3.4 is very flexible and retails around $200 for a 10ft factory terminated pair.