You can use yum-config-manager to add an online repository provided you have the url to a online repository (specifically, the .repo file):
yum-config-manager --add-repo repository_url
yum-config-manager --enable repository
repository_url is the url of the .repo file and
repository is the unique repository id that can be found using the
yum repolist all command.
I believe that as long as you have a RHEL subscription, you are guaranteed access to RHEL repositories that you can access via yum. According to page in the RHEL7 documentation,
When the system is registered with Red Hat Subscription Management to the certificate-based Content Delivery Network (CDN), the Red Hat Subscription Manager tools are used to manage repositories in the /etc/yum.repos.d/redhat.repo file.
So if you want to access RHEL repos, you will need to use those tools. You can find more information on them in the RHEL documentation online (Documentation for RHEL 7, System Administrator's Guide, section 5.2: MANAGING SOFTWARE REPOSITORIES). For RHEL 7, you should be able to do this:
subscription-manager repos --list
subscription-manager repos --enable repository
The first command shows what RHEL repositories your subscription affords you, and the latter enables one of them.
Based on this output:
test:~ # subscription-manager repos --list
This system has no repositories available through subscriptions.
It appears that your system does not have an active subscription. If you want to obtain a RedHat subscription, you will need to register with RedHat and purchase one (link in the comments).
If you do not have access to these repositories and would prefer not to buy a RHEL subscription, you can always install a CentOS repository, as the two are built from the same source and therefore their packages should be compatible (use your own discretion here, though; RHEL repositories are preferable). You can determine which release of CentOS is compatible with your particular installation on the CentOS wiki page (downloads section, towards the bottom; yours should be CentOS 7 (1511)), then use the "tree" link to the right to access a collection of rpms.
You can find more information on CentOS repositories on the CentOS wiki page under Additional Resources -> Repositories.