In The Silence of the Lambs, if Hannibal already knows Buffalo Bill and was willing to help the FBI, it seems easier to use a sketch artist and then release the photo to the public to find Buffalo Bill. Why didn't they do this?
Why didn't the FBI use a sketch artist to find Buffalo Bill?
The question assumes that the FBI have someone who is willing to describe Bill to a sketch artist... and they don't.
Yes, Hannibal knows Bill, or at least could describe him, but he has no particular reason to tell the FBI that he does and less to help them any more than he is willing to do.
Hannibal only "co-operates" with Clarice for his own amusement and to play mind-games with her and the FBI. He doesn't really intend of helping out unless it benefits him and he resorts to trickery to be placed in a position where he can escape.
"Two things to begin with. First, we go on the premise that Dr. Lecter really knows something concrete. Second, we remember that Lecter looks only for the fun. Never forget fun. He has to want Buffalo Bill caught while Catherine Martin’s still alive. All the fun and benefits have to lie in that direction. We’ve got nothing to threaten him with."
Jack Crawford to Clarice Starling - Silence of the Lambs - Thomas Harris