There are a variety of resources online for finding the IP blocks allocated to countries (generally for blocking purposes). Assuming you have adequate hardware router capacity or are running your routing on an adequate PC, you'd need to get the IP blocks for your country, set up a hardcoded remote access address for yourself, then set up rules to allow connections from IP blocks assigned to your country while blocking everything by default.
Unless you have physical access, set up your bypass first - it would stink to discover that you made a mistake in your permissions and the only rules being applied are the "block everything" ones.
Finally, if this is really a DDOS, blocking at your local firewall/router is probably not going to be terribly effective - a big part of the effectiveness of DDOS is not that it clogs up your servers (which it does) but that it clogs up the Internet connection from your ISP, or even the connection to your ISP. Filtering of the sort you may require would have to be done "upstream" from your connections.