Sorry, it doesn't look like there is a way to stream it on subscription. https://www.justwatch.com/ca/tv-show/elementary
It's showing as buying from Apple as the only option (without purchasing a physical copy).
For everyone asking themselves if old Moriarty will be back, this is the description from trakt.tv for the last episode: "Amidst Sherlock and Joan’s battle with tech billionaire Odin Reichenbach, the investigators receive word of their old enemy and Sherlock’s former love, Jamie Moriarty."
If you're in the US, it should be on Hulu.
For other countries (and the US), check JustWatch - it shows which providers offer a particular movie, or TV show (by season).
PS: It's a great episode.
It should be out in all regular bookstores in Australia this week (not sure of the exact date, might be Friday 27th February, which is the UK pub date), but you could also try The Book Depository if you want to order online.
If this site (http://www.tvmaze.com/seasons/85032/elementary-season-7) is accurate, only 13 episodes have been ordered for the 2019 Season 7. All previous seasons have had over 20 episodes. This could be the beginning of the end.
I just looked (2019-01-05 23:38 ET), and can't find any season of the show on Netflix.
According to JustWatch, it can now be streamed from Hulu:
https://www.justwatch.com/us/tv-show/elementary
P.S.: It could be worse, it could be exclusively on CBS All Access. 🙁