Are there any national/global online databases or is this more of a regional business since actors can be flown in, but most people are extras who have to be "sourced" locally?
The thousands of casting agencies in the world have headshots of actors they work with, as well as databases with their stats: gender, age, height, weight, hair color, eye color, skin color, fitness level, etc. If a producer needs a specific look, they will usually work with a casting agent. Based on the criteria that the producer gives them, the casting agent can quickly narrow down their list of actors (only females, with long brown hair, between the ages of 30-40, etc.). The casting agent can send the headshots of just those actors to the producer. The producer can then quickly look through the headshots for a particular appearance.
Note that each casting agent represents a completely different roster of actors. If a producer needs a very specific look, but he/she doesn't find an actor matching that appearance in the headshots from one particular casting agency, the producer may approach another casting agency and ask for their actors who fit those particular appearance criteria.