"Look, I can't help you," he started, "but I can find someone who can."
The sentence was interrupted, but if you put both things in quotation marks together, they make one sentence.
There doesn't need to be a comma after the description of who is talking if the next line of dialogue is not a continuation. For instance:
"Look, I can't help you," he said. "I simply can't."
See the difference? In the first instance, there was one sentence being interrupted; in the second instance, there were two sentences