ISO 3166 is a standard published by the International Organization for Standardization (ISO) that defines codes for the names of countries, dependent territories, and special areas of geographical interest.
The ISO website contains a searchable and sortable page with all the official country code assignments.
The standard allows for decimal 900–999 to be allocated privately. The guidelines governing the Allocation of Service Information (SI) codes for DVB systems (TR 101 162) allocates these to regions that are useful for the allocation of Network IDs and Bouquet IDs for satellite broadcasting.
The table of country codes is accessible via the button above.