A "Backup Battery Low" message is an indication that the internal battery is running down. The internal battery is designed to save custom/user settings when the power is turned off.
The battery was a model 2032 (watch/calculator type) and can be found at most all electronic supply stores that have a battery section. Note that due to the age of the product, we do not have a specific resource regarding where to find these.
Instructions for replacing the battery can be found on pages 395-396 of the Owner's Manual.
Note: While it will not cause harm or permanent damage to the VS-2000CD if the battery runs down, ALL User settings will be lost when replacing the battery. Be sure to backup any user settings you wish to keep before replacing the battery.