Better TweetDeck was available on https://addons.opera.com, but the Opera add-ons team's review times have been very complicated to deal with, so the Opera version always was days if not weeks late compared to the Chrome/Firefox one.
Fortunately, there's a "better" way. It's a workaround, it's not ideal, but at least you'll get updates at the same time as Chrome and Firefox users!
Follow those steps:
Feek free to reach out on Twitter @BetterTDeck if you run into any issues.