They are many chooses to add ( download) and start a box in vagrant:
1- boxes from bento project ( yes like the boxes 
)
2- Vagrantup website
1- boxes from bento: https://app.vagrantup.com/bento
( and it's github https://github.com/chef/bento )
C:\Users\jkriker\Documents\vagrant\myfirstproject>vagrant box add bento/centos-7.3
==> box: Loading metadata for box 'bento/centos-7.3'
box: URL: https://atlas.hashicorp.com/bento/centos-7.3
This box can work with multiple providers! The providers that it
can work with are listed below. Please review the list and choose
the provider you will be working with.
...
C:\Users\jkriker\Documents\vagrant\myfirstproject>
2- Vagrantup website
...