These links get you complete STI disassembly:
http://www.norvil.net/pedal/service/...ti/removal.php
http://www.norvil.net/pedal/service/...9sti/strip.php
This one gets you reassembly:
http://www.nitrodyno.com/Ultegra-STI/index.html
The problem is finding appropriate spares. Cannibalized levers are the only choice, and it may be a crap shoot as to what you're getting for parts unless it was your lever and you know the history.
Campy does it right in this respect. I just overhauled a pair of Record levers w/ new g springs and carriers in under an hour and they shift like new. I'll do it again in 8,000 miles or so and they be good as new for another cycle. Cost was <$20.