Hello developer, I asked myself the same thing some time ago when I thought about creating my own project, but remember that its developer received a
DMCA lawsuit for containing Nintendo material, and for that reason it only contains the
code used for the project, but not its graphics for legal reasons.
Back to the topic, it is not impossible to find its graphics such as tiles and sprites, since the sites focused on the preservation of these curious archives. In my little search I found the sprites and some tiles of
Pokémon HeartGold and SoulSilver, which is the most used by the community, there is also the
Diamond and Pearl one, and even the
Black and White ones, but unfortunately it is not as complete as Essentials was in the past.
Now talking about other resources like music, I must assume, It can be easy or difficult to find, you just have to search, also recreating the music wouldn't hurt.