In summary, any MIDI keyboard with at least 49 keys will be suitable for learning piano with Musiah.
What is a MIDI keyboard?
In practical terms, a MIDI keyboard is one that is capable of being connected to a computer or iPad.
To find suitable examples, simply google “buy midi keyboard 49 keys USA” or similar (e.g. if you live outside the United States, replace “USA” with your country-name).
To learn more about different kinds of MIDI keyboards, view MIDI Keyboard Requirements.
Alternatively, if you don't own a MIDI keyboard and you would like to try the first few lessons in the Musiah app, you can set up your computer keyboard to play piano notes.