Make a note of the packages that
yum wanted to download. Use an Internet connected machine to download those packages. Place them on an CD/DVD/USB-stick and import them into your CentOS machine.
cd to the directory where you've imported them and run:
yum install <package-1> <package-2>
I did a quick test on my CentOS 7 machine and the packages consisted of
tftp-server therefore it should be easy enough.
Another option would be to download the full ISOs of CentOS and burn to DVD. Then configure the CentOS machine to use that as a repo. Bear in mind that the packages will be relatively dated (as I write, they are 9 months old).
Finally, you could
rsync a repository's
repodata directories onto a large USB-stick or a USB HDD, then mount this on your CentOS system. Create a
.repo file that points to this mount and install
tftp-server. This will have the advantage that the package will be the latest available. You could even
yum update the whole server.