Musterbrand has some gaming related clothing, not much, but I guess they ship to Italy.

Musterbrand

These should still be an option for you:

TeeFury ships to Italy

The Yetee ships to "most" countries, check with them or try placing something into your cart without checking out to perhaps see shipping option to your location

OtherTees ships anywhere - see pt. 9 in their FAQ
EDIT: This company is in Poland.

Shirt.Woot ships to Italy

Shipping cost seems reasonable at all of these.

Yeah, I was checking them, but it's still an extra cost + time + pain in case of problems.
(also, where they say how much is the shipping?)
Still, +1 for the suggestions.

I'm going to buy myself a Batman t-shirt for my birthday, or for christmas. Perhaps I'll look for some batman swimming trunks while I'm at it

Long ago I visited a website superherostuff.com
they have few tees which might interest you.

A great place is Redbubble.com. They sell user created designs of anything, you can even sell your's there, and tons of gaming shirts. The shirts are soft and feel great. They ship from either the UK or USA, depending on the destination.

This is one of my favorites, targeted to the automobile enthusiast crowd:

www.blipshift.com

Not sure their methodology will work for your needs. They make one shirt design at a time, gather the orders for 3-4 days, and then have them made to those exact quantities - I get my shirt about two weeks later. Every once in a while they'll pull old designs out of the 'garage' and offer 10-20 shirts at once. Those are likely your best bet for international orders, since who wants to pay int'l shipping on one shirt at a time?