I think at the moment, your only option for sure is his fixed meeting spot in the Fantasyland theater, it's his classic black/red/white/yellow attire.
Other options to see him are:
- The Stars on Parade - Steampunk Mickey
- Shine Brighter show / waiving at everyone from the train station in the morning - 30th anniversary outfit
- Mickey and the Magician show - Magician's assistant.
After the 1st:
- Halloween parade: spooky outfit.
- Not much has been released yet for the Halloween season, but it does say Mickey will meet. Possible places: Phantom Manor, Main Street (usually during EMH), Frontierland (happened in the past, not since Covid)
The Phantom Manor meeting spot can be used for either Phantom Manor Mickey or for Jack Skellington (black & white outfit)