There are endless sources for all types of media, some can be more useful or informative than others. Whilst this list isn’t exhaustive by any means, it could provide some helpful pointers to where you can find new or old information.
If you find an article or source in a language you can’t read, linking it for moderators to look at is still extremely helpful.
In most cases, official sites are the best possible source and contain the most information. Providing these as often as possible is ideal. Especially since these sites should be linked in the External URL section of the form.
Sources in the native language are extremely helpful - even if you’re unable to read it. Searching the native title can bring up relevant news articles or, for a lot of digitally published works, sites where they are legally hosted (Example: PixivComic, Nico Nico Seiga, and ComicWalker).
News sites are usually fine, but double-checking the sources they reference can be useful.
Media Arts Database is an amazing resource managed by the Agency for Cultural Affairs’ for all manner of information, from animated series, to manga tankoubon, to print magazines dating over fifty years old. Always worth taking a look to see what’s there. (Examples: Dennou Coil, Weekly Manga TIMES)
Please remember that magazine issue date is not always the month the magazine was released. Many issues release a month or two earlier than titled. Checking on-sale dates on the magazine's website or sites like Amazon is required.
AniDB is very good at tracking staff roles, but names should always be double-checked for accuracy. (Example: Ginga Eiyuu Densetsu)
Pixiv Encyclopedia can often be useful for not only character names, but all manner of information, similar to Wikipedia, a lot of information gets collated into series pages. (Example: 3gatsu no Lion)
MyAnimeList, MangaUpdates, and Kitsu are often sourced - whilst this is OK, these alone aren’t always accurate enough, as they can often contain errors. However, please link the first two if they exist, especially as AniListrequires the works' MAL ID for mapping series between the two websites for list imports.
Anime and manga aggregators (streaming/scanlation hosting sites) should almost never be used as a single or primary source. They usually lack or have incorrect data.