I’ve been researching this for a few months. I found a lot of the films on youtube and archive.org. Some of the ones I have are incomplete. Quality ranges from poor to great. I tried to get the best picture quality and longest length films. Some are completely silent, some have fake background music and some have film projector noise. One film has the captions in another language.
I used the following website for the complete known filmography:
I felt this was the best site since they have the exact dates the films aired along with descriptions of them. In youtube, there are some wrongly named films. Using this guide helped me rename them to what they really are. There are also a lot of untitled snippets of Felix the Cat films on youtube. Using this guide helped me determine if they were new films or repeats.
I found a lot of good quality films by searching youtube for felix the cat in Japanese: フィリックス・ザ・キャット. There was also a show on Cartoon Network Japan called "Late Night Black and White. I found a rare felix film on one of the episodes uploaded on archive.org. There might be more hidden.
I also have some alternate versions of films. They are usually shorter than the ones I kept but some of them have added sound/music. Some of these films have changed scenes. I am thinking they were rough drafts. Some are colorized.
This website has a lot of info on Felix the Cat 9.5mm films released around the world.
You should check with your sources to see what they have before searching online. I may have some stuff they are missing but I doubt it.