Is it ethical to obtain journal articles from places other than the publisher's official sites?
Yes, it is absolutely ethical to use these files during your research. Many publishers allow academic authors to upload a so-called "preprint" version of a paper to their own institutional websites or put them in repositories, and of course these can be used by others in research.

There's maybe just two minor things to be careful about:


You should try to verify that the preprint version is mostly up to date with the published version, comparing e.g. publication dates.
It is common practice to put the "official" publication source, i.e., the journal version, into your reference list. Nevertheless, if you can't make sure that the versions are equivalent regarding what you're citing them for, it may be necessary to state that you were using a preprint version.


In some cases, authors may put papers online even though it's technically a breach of copyright. But since you don't know the author's agreement with the publisher, you have no way of checking that, and in any case, it would be the author or institution that violates copyright, but not you.