although you could just use a plain 'ol Campy 9 cassette (and chances are it would shift OK or better), your best bet is to get a loose-cog Campy 9 cassette (such as a Veloce), and fit this spacer kit: http://www.branfordbike.com/cassette/cog11.html#item2
now, in theory, you might be able to just take the spacers from a 105 cassette and use those, but I’m not sure if Campy cogs are different in thickness from Shimano cogs. don’t try that without checking out the cog spacing and thickness first.
Use a Campy 9sp cassette. Will work just fine as is with Shimano 9sp. I run this and it works fine. If you need reassurance, do a search on Google groups, rec.bicycles.tech. Plenty of positive references to the same thing.