The campy compatible casettes steve talks about are only necessary for 10 speed and the ones they have over here cost a fortune.
Since you want to use it for 9 speed campa it won't be much of a problem. Campa and shimano hubs only have a small difference, campa is about 2mm wider.
You should be able to use shimano casette with campa shifters.
If original shimano casette doesn't work you'll have to try a few times, change a part up to all of the rings between cogs(remove shimano ones insert campa).
I Have a bike with Shimano ultegra(9-speed 1999model) shifters and one with chorus(9-speed 2000 model).
I have mavic Cosmic Carbons wheels with shimano body and Dura ace casette with original rings wheel works fine in chorus bike without any problems.
I've got another wheel where i've had problems on the 3 last cogs using a ultegra casette, I repalced 2 shim rings by campa and the wheel now runs smooth with my Chorus.
So you wil have to try out jourself or get somenone handy to see if or many rings you have to replace to make it work smoother. When having problems with the cogs combined by 2 and 3(where you can not replace the rings, you can ad a small ring between the 2 and 3 cog combi's(there are small metal rings from the 8-speed erra under 1mm in width who will just do fine)
So far the cheap solutions but those who require qome handyness.
Other solution I think your freewheel is replaceable(those of my mavic CC's certainly are and I believe all mavic's are(check bikestore for ksyrium) so you can get your shimano freewheel replaced by a campa one(which is off course perfect for campa).
There is a small chance when you enlarge the with of casette(by rings change or freewheel change) the cogs got too far to the right. Then you willhave to adjust(or let adjust) your axles(isert spacer.
Don't be scared Freewheel cahange is the biggest operation. But for people with the right tools and handyness its doable.
I adjusted 2 wheels(from shimano 9 to campa 9) in about 20 minutes
IN BRIEF
Original casette might do, if not try one off the above, feel free to ask when unclear.
