In OpenTTD, cola is the flat version of the popular drink of the same name. It is obtained from cola wells, which due to their mysterious and somewhat comical-looking nature of course only appear in the toyland Climates. The cola is then transported to a Fizzy Drink Factory in order to have the bubbles put in and thereby be made into the more recognisable sparkling beverage.
Being a raw ingredient, cola is transported by tankers - it is only canned at the Fizzy Drink Factory.