On top of Strikes, Crucible, and Gambit, you can also now get it from Vex Offensive.
Edit - Quote and link.
[quote]Rewards
The Season Pass weapons Pluperfect and Temporal Clause now have a chance to drop from Vex Offensive after you have obtained them.[/quote]
[url]https://www.bungie.net/en/Explore/Detail/News/48420[/url]
Level up your season pass until you have the chance to get it to drop from the crucible, strikes or gambit, then just wait and pray haha.
Alternatively starting today, those weps from the season pass will drop from vex offensive as well.