Here's the full entry from Oxford English Dictionary:
Pronunciation: Brit. /ˌrɛfəˈrɛndəm/ , U.S. /ˌrɛfəˈrɛndəm/
Inflections: Plural
referendums,
referenda.
Etymology: < classical Latin
referendum , gerund or neuter gerundive of
referre refer v., probably after French
referendum (1781 in sense ‘demand for consultation’; 1874 in sense
1; 1877 in sense
2). Compare earlier
ad referendum adv., and also
referendary n.Th
e plural forms referendums and referenda are both found; in the early 21st cent. usage is fairly evenly divided between the two, as it was also in the late 20th cent. The form
referenda is by analogy with
memoranda ,
agenda , etc., and more generally with plurals in
-a of Latin-derived words with singular in
-um . This form is sometimes deprecated in usage guides, etc., on the grounds that a Latin plural gerundive
referenda , meaning ‘things to be referred’, would necessarily connote a plurality of issues, but this view is unlikely to affect actual usage
It seems referendums is a okay.
Language changes over time, which is why it sometimes bugs me that we get so hung up about correct spellings, texting etc. I have to say I am a nitwit speller, so I'm probably biased!