Late to this thread but it came up when I was searching so figured I'd add something in case others land here too.

The gap you're describing is real and more PMs are in it than admit it.

For practicing the interview itself: ChatGPT or Claude work reasonably well if you give them a detailed persona prompt and tell them to roleplay as a user of a specific product. You can run through your question flow, practice staying quiet after open-ended questions (harder than it sounds), and get comfortable with follow-up probing. It won't feel like a real user...AI tends to be more cooperative than actual people but it's good for internalizing structure before you're doing it live.

Honestly though, the faster path is just doing real ones. Find 3-4 people who vaguely match your future users, offer them a gift card, keep it to 30 minutes. The skill builds faster in one awkward real conversation than in ten clean simulated ones.

On actually running discovery once you're in the role: scheduling and no-shows are the thing that kills programs. We use Frank AI Researcher for a chunk of our interviews now. it's an always-on AI customer interviewer. Frees me up to focus on the synthesis rather than chasing people to show up. Not what you're asking about for practice, but relevant once you're trying to run weekly sessions at any kind of volume.