I don’t think Campagnolo themselves sell individual sprockets (and if they they did, they’d likely be prohibitively expensive). I think your options are either (1) buy a Veloce cassette, which comprises individual steel sprockets, or (2) buy an individual sprocket on eBay. A 10sp cassette will cost you ~$45, while individual sprockets run $30-35. You might find NOS 8- or 9sp cassettes for less