You're probably already aware, but most levels are on the level database, with author and information, and thanks to Akseli many of them even with ratings:
http://lemmings-db.camanis.net/levelpack/list(I don't really like Fun/Tricky/Taxing/Mayhem as ratings/sortings, as the original stages contained mostly levels not only of a similar difficulty, but also of similar style, and nowadays many levels tend to be hard than Mayhem as well.)
I packed a whole bunch of levels, unsorted, a while ago into an archive, if you want to download a lot of levels in one chunk:
https://dl.dropbox.com/u/43603680/levs.7z Some of them aren't in the database.
I'm not aware of anyone designing level not in Custom Lemmings style, so I guess it's safe to assume they are unless it's levels directly from e.g. ONML.