Sometimes there's a mod that you just can't find on CurseForge, what do you do?
It's entirely up to the author of a mod to upload it to CurseForge, however, if the mod is hosted publicly, has a proper license that allows us to distribute the mod, and you can provide us that information we're happy to add it to our Approved Non-CurseForge Mods list.
We recommend first checking the list to make sure the mod you wish to add isn't already on there, and if not you can submit the information for review - Submit a request.
It can take several days for a review to occur, but once it has you will be notified with either a message declining the addition or approving it along with any information as to why or why not.
If a specific version of a CurseForge-hosted mod is not on the site, this means the author has chosen not to upload it for some reason. These versions are not allowed to be included in modpacks.