Does Rei's favourite drink exist in real life?
While the in-work name is fictional, it is based off of real products: jelly drinks. More or less, all jelly-type drinks are in a semi-hard bag with a mouth to drink from on the top. And most of these drinks usually serve some health-oriented purpose, quick energy boost, daily vitamin intake, etc. There are many jelly drinks, and one that is very popular (and most likely the model) is the Weider drinks.

Here's the website for Weider's jelly products.