On the Spares - Request Parts Screen there is a magnifying glass that if hovered over, says 'Cross Site Lookup'. If you change your filters to 'All Sites', you can look up spare parts available at other facilities on your server. When you click on the magnifying glass for a particular spare, it will bring up a box with a list including that spare and what sites it's available at.
Note: the Default Search Option is set to 'Current Site'.