You will need to get creative. Find a friend who has internet, and use a USB memory stick to move the stuff from there to your machine. If I were you I would get every app you think you must have all at one session on your friend's machine.
If you have an android phone you can download to it, use the USB charger cord to connect the phone to the linux box, then move what you need from phone -> CentOS.
A lot of linux support sites will send you a CD/DVD with everything on it. Costs are small, but it takes a while to get the CD in the mail see this website on somebody else's machine:
Do you already have the packages for the repo somewhere? You'll need to download, or otherwise obtain them from somewhere.
Once you have the packages, you can create a repo from them (createrepo command) if the metadata was not already created. Then you need to add the repo to your system by adding it to somewhere in /etc/yum.repos.d.
What you do first depends on whether you already have the packages somewhere.