At first estimate we could just Google images of polluted rivers in Asia and Africa and look at the logos on the garbage.
From my priors, I'd estimate that the plastics would almost all be from locally-consumed products, simply thrown into the river as is common and accepted in such places.
My sense is that a lot of educated, wealthy Westerners don't realize:
1. How many people are in Asia and Africa.
2. How little effort they put into keeping their local environment clean. People just don't get how callous same cultures are about the environment; it cannot be believed until it's been experienced since it's so far outside of the wealthy Westerner's experience. (Plus, to believe such would also create cognitive dissonance with social justice creeds which are victim classes as inherently morally pure.)