To find a diagram of the 2003 Ford Focus serpentine belt, you can either go through image libraries or specialist photo stockists, or seek the assistant of the online motoring community.
I used Google Images to find this picture of the serpentine belt of the 2002 Ford Focus serpentine belt:
My understanding is that the serpentine belt arrangement is similar between the 2000 to 2005 Ford Focus 2.0l range, though of course your model may not have the A/C option fitted.
Where Else to Look For Your Diagram
There are several online sites who can help you out. For a start, there are general sites offering advice on request on car-related queries, such as fixya.com, 2carpros.com and greatautohelp.com.
Alternatively, you could head for more specialist sites such as serpentinebeltdiagrams.com or fordfocusfanatics.com. The latter is an online gathering of Ford Focus drivers you could help out with advice on spare parts and where to find a copy of the driver's manual if you have mislaid your copy. Join up, post a question and start the discussion!
Lastly, try Ford themselves on their customer technical helpline, 020 3564 4444, option 4.
For tips on how to change your Ford Focus serpentine belt, watch this YouTube film. Remember if you do change the belt yourself, remember to take a picture of the belt before you start for later reference.