I was going to suggest using Jxself's apt repository, but I just read that you use Manjaro.
It seems like the Linux-libre kernel is in the AUR. I don't use Arch and have never used Arch-based distros, so I can't say how well it will work.
If all else fails and you are insistent on using the Linux-libre kernel, you can just install a 100% libre distribution. There are ones that are Arch-based, if you so desire.