Passport stamps are certainly collectable, but they're not listed in the regular catalogues, which means that I'm not sure how your stamp could be ID'd.
There may be a very specialised Italian-language catalogue which lists all kinds of issues, including passport stamps, but I don't think so.
The best online guide to Italian stamps doesn't list them. This website can be found at this address:
http://www.ibolli.it/index.phpA suggestion would be to search
ebay Italy for 'Marca Consolare' and see what comes up.