it’s a process of building your own database and expanding on it with multiple sources. It also depends on which language(s) you are collecting.
- pkmncards is great for quickly looking up english releases
- pokeboon has information on niche JP promos
- Here’s a comprehensive mewtwo poll that will have most if not all releases
Mewtwo is a popular pokemon so I imagine someone here is also a mewtwo collector and can help you out