My brain is full of opera this month- I’m playing on an opera program, subbing for a rehearsal, and preparing an opera audition.
I think I’ve actually started to prefer opera over the symphonic rep…
One of the things that really improves my workflow is to spend time organizing at the outset. I don’t love this part- I would rather go straight to practicing.
But it pays off in the long run, especially when I have a lot of music to prepare. This way, I don’t have to go searching for something in the middle of a practice session.
Here’s what I use to prepare all this music:
IMSLP: obviously! I have a subscription, and use the (free) app, since I download so much music from there.
To prepare a rehearsal, I study and play through with both the score and the part, with a recording.
I also make set lists on the app, and use the bookmark feature to mark sections, especially in the score. Those opera scores take FOREVER to flip through!

iTunes: I make myself a playlist of everything I’m studying to listen while I drive, and to have it organized when I’m practicing.

Wikipedia: For background on the composer, the opera, and the synopsis.
Opera-arias.com: To read the libretto and translation. Also, to find a particular aria within the opera.
Arias are titled by their first line or so of text, but in the instrumental parts, are often only designated by a number. In the case of a through-composed opera (Wagner, Puccini), there is no number, and you have to sift through the score!
Musescore: Free music notation software, which I use to make my own arrangements for the practicing/studying. I also use Musescore for all my other arrangements.

One thing I’m really excited about is that I am playing 2nd flute on the famous Entr’acte to Act 3 of Carmen. Which means I get to hear the solo up close and personal with an orchestra!!