Referendums or Referenda?
Referendums. End of story.
The word "referendum" is an Anglicization of a Latin gerund. Which means, by definition, it has no plural. So when you use it as a noun simply follow the standard English practise of adding an "S".
The world "referenda" is used by intellectual snobs who think English should blindly (and in this case incorrectly) follow the grammatical rules of a dead language. The oft quoted commandment "Thou shalt not split infinitives!" is another example of this nonsense.
Having learned Latin I can honestly say the English language looses nothing by ignoring the rules of Latin grammar.