The VL-1220/VCN is the specific RTC battery backup for the BeagleBoard.
If you can’t get it from Digi-key (ie because minimum shipping is $50) then you seem to be out of luck.
This battery allows a BeagleBoard owner to keep the date and time set after power off.
If SF stocked this I would imagine a large percentage of orders for BeagleBoards would throw in this component too.