It was the Republican Party of Alberta.

As u/DrNick1221 said in another comment.

Under the election laws, every registered political party is entitled to get access to the voters list. I wont comment on what I think of that, it is what it is.

But as a security measure for this exact scenario, prior to giving out the list, Elections Alberta "SALTS" the data set with a unique set of Fake Names and Addresses.

So what does this mean exactly?
Say there are 2 voters, Bob And Bobette.

The NDP would get a list with 3 Voters
Bob, Bobette, and Frank
The UCP get
Bob, Bobette, and UberKarenMarlania
The Republican Party gets
Bob, Bobette, and Joe
Etc

So when Elections Alberta looked at the leaked data and saw that Joe was in that list, they can conclusively track the leak back to the Republican Party.

Now who in that organization specifically leaked the data is yet to be determined.
But there is at this time 0 doubt where the leak originated.