If you are wanting an efficient way of locating heads, torso or legs for purist custom minifigs then a great way to search for them is to use goatleg, which creates customised searches at bricklink depending on what you want.
Not only do you get to see the list of heads, torso or legs based on your criteria, but it is also simple to find out which set / minifigure they appear in and what the secondary market price is on bricklink.
To do it:
1) go to http://goatleg.com/
2) select head identification (or torso or legs)
3) under Minifig Heads by Pattern, select colour and stud type (if needed), then select from the pre-determined facial features, etc or just enter a keyword for your search
4) Then either select list with images or thumbnail gallery (better for visual searching) and you will see what you customised search brings up.
5) Click on the part number for details about the part, which set it comes in, which minifigure it comes in, its current price, etc.
It is very flexible (search for what you like, filter by colour, etc) and it is (nearly always) up to date being based on BL's catalogue.