Hello,
Get a free GeoIP database http://dev.maxmind.com/geoip/geoip2/geolite2/ and start building a prefix-list/address-book entry.
But I'd reckon it is going to be very long unless You'll find a way to automatically aggregate the individual entries, should be possible with a script. But You'd need a script anyway to periodically refresh this database from Maxmind website.
HTH
Thx
Alex