First of all, your description is awfully vague; a picture would help a lot.
Secondly, Just Cavalli is not held in particularly high esteem around these parts, so it's doubtful you're going to find a local expert on the brand -- though maybe on another forum.
That said . . . .
Very few things get made more than once. For most designers/brands, a particular item is made for a season and then disappears. If it's extremely popular (i.e. a money-maker) or it's central to the brand aesthetic/identity, it may be reincarnated -- usually with subtle variations, especially if it's a piece that recurs in many of their collections.
My point is, if you are after a specific shirt and you've been looking "for a long time" (I'm guessing that means more than a month or two), it's probably out of production and long gone. Keep your eye on eBay and Yoox.com -- they both offer long-lost products from time to time, and those are pretty much your only options at this stage.
If it's a fairly current collection, you might still find the shirt you are looking for on a fashion product aggregator such as
ShopStyle.