Good quality serpentine belts i have bought have been Gates & QH (Quinton Hazel) branded. Not so good quality serpentine belts were Dayco & Continental branded. The Dayco, a 6pk2080, failed and decided to change itself into a 5pk2080.
Never bought a kit just the belt on its own. UK based motor factor partsinmotion.co.uk are my go to supplier.