Jordi Boggiano and Nils Adermann, creators of Composer, have recently released a paid version of Packagist. The service aims to make managing private packages a breeze.
Private Packagist aims to remove all these hurdles for businesses to finally make working with Composer as convenient as it should be. Being a hosted service, setting up your own Composer package repository on Private Packagist is done with a few clicks. No matter if your private source code is hosted on GitHub, GitLab, Bitbucket, any of their on-premise solutions, or in any other Git, Mercurial, or Subversion repository, Private Packagist can immediately access your code after setting up your credentials to make it available for installation through Composer.
If you're not afraid to get your hands dirty you could, instead of using Private Packagist, choose to use Satis. This tool is also written by Jordi & Nils. Laravelista has posted this great tutorial to get you started with the tool.
At Spatie we have set up a satis server to register packages that are intended to only be used in our own projects.