I will add that Cannondale measures their frames slightly different than some other companies. At 6 feet tall I ride and fit a 56 cm. Cannondale frame better than a 58, although both sizes will work for me. But a Trek or Specialized bike has to be a 58 which fits very similar to Cannondale's 56...
I have a very similar looking 2005 Caad 8 R5000 with Dura Ace components. You can find almost anything on EBAY if you have enough patience to track it down. Including NOS (new old stock). At the very least a near new set of shifters should be available. Good looking bike.
Your bike is in fantastic condition. I have a similar bike (1989 3.0 Criterium) in similar condition with a mixture of Dura Ace, 600 Tri color, and 105 components, all original, and have often wondered how much it is worth. I still ride it on the trainer in the winter and a few times every year...