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 (here: https://docs.google.com/spreadsheets/d/1lEwbJpwRVoZb8kr0OT-Wo4Yc7jAGWMGOFtfL0miCFmc)
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 here: https://docs.google.com/forms/d/e/1FAIpQLSf5L4DKT8nPZwg9nyeIsnp0mh3D_a8ctbK_bLXgnZBqUre6AA/viewform
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.