Yes, you can get protein powder at several places, pharmacies and such. Aquarius Fitness Center on the West End (in our Fitness section of our Things To Do) also sells the name brand fitness protein powders. They also have an amazing variety of machines and weights, all in an open air setting.
There are many affordable Bullet-type blenders available at stores including pharmacies and haberdasheries. You could use it for your stay and leave it on the island for someone.
Right next to Travellers is 1 Fusion Negril. For decades it was called Errol's Sunset Cafe, for a short time Sunset West and now 1 Fusion. They are open 24/7 and a very local vibe. Still family owned and operated.
Big Roy at Cafe Goa likes to dark roast his High Mountain beans (picked on the Blue Mountain but does not have the official "Blue Mountain" stamp, just like Thunder's). Its a personal preference but I always liked Roy's beans over Thunder's.