the scientists that made the 8.3 billion straw estimate are Australians Denise Hardesty and Chris Wilcox.
https://www.sciencedirect.com/science/article/pii/S0308597X15002985

Warning, the above is a very long and boring academic journal, but i suppose that adds credibility to it. One of highlights of the journal is that nearly 50% of the actual plastic waste in the oceans is discarded fishing nets, which I can promise as an ex scuba instructor, kills far more sea turtles than straws.