Hi Alex,
I've recently come back to flight sims & now running FSX as well. Adding 3rd. party repaints to stock FSX is not that difficult, actually a bit easier than it was in previous editions of Microsoft FS. Just download & unpack to a directory you should make for FS downloads (I have several sub-directories for cockpit panels, engine sounds, etc.). Once unpacked, place in the appropriate simobjects\airplanes then look for the specific aircraft your adding subdirectory (kinda long path for me because I run FSX through Steam - C:\Program Files (x86)\Steam\SteamApps\common\FSX\SimObjects\Airplanes\a321 - for example). For repaints of a stock aircraft all you need is a texture file and updated aircraft cfg. file that is usually included with the download. Once you cut/paste or drag/drop the texture (needs unique name.. ie - texture.alitalia) on the list with the other textures cut/paste the included aircraft.cfg entry onto the existing stock FSX aircraft.cfg (using notepad) at the end of the current list of specific liveries. Each (livery) texture start with the #0 then 1, 2 and so forth. The included downloads will come (usually) with a aircraft.cfg that has x for a texture #. You need to change the x to whatever the next # on the list of the aircraft.cfg you are adding to. In the rare case a texture does not come with a aircraft.cfg you can make your own easy enough by cutting/pasting another aircraft.cfg entry that is on the desired list & change the title, the name of the texture and # to the one you are adding and place at the end of the aircraft texture list for a particular model.
As has been found by others there are repaints your looking for but Alitalia didn't fly the 747-400 or the 737-800 (I looked..
https://en.wikipedia.org/wiki/Alitalia) not that it matters.. My prudish nature I only want aircraft a particular carrier actually flies or flew previously. If you merely type in alitalia in the Simaviation search bar you get pages of aircraft Alitalia currently fly and those they have in the past. Most of these are complete aircraft, meaning that you can download the aircraft & all you have to do is once unpacked add to the ...\simobjects\airplanes list. Though many of these aircraft will default to sound or panel files of stock FSX aircraft some come with their own unique ones. Many aircraft you add you'll also have to amend other FSX files (sound, effects, etc.) for them to work right, no worries these download almost always come with directions how to place them in FS correctly. Unpacking beforehand helps prevent problems and be careful where you put things. For example in the FSX\effects sub directory there is a sub-sub directory called texture (ie FSX\effects\texture). Some aircraft you download will have to move additional files to both the effects sub-directory and the texture sub-sub directory it contains within.
For Airbus you can't go wrong with Project Airbus files,
http://www.pafs.wf/downloads and Project Opensky has a wide variety of other models
http://poskyarchive.com/ which though designed for previous models of FS can be made to work with FSX with many of these are available here on Simaviation without going to those other sites. For additional aircraft just check the description of aircraft you might be interested in on the Simaviation search list. One thing to remember is you cannot mix models from different developers on your aircraft list. Some aircraft of slightly different makes from the same developer can be in the same directory say if they have different models of the same aircraft, usually reading the directions for particular downloads clarifies where things have to go. This is the case with aircraft from Project Opensky aircraft. One last note when adding aircraft. Merely adding a new texture & updating the .cfg file is all you need when adding to stock FSX is to switch to another type of aircraft and switch back for the additional livery to appear. If you add entirely new aircraft with new sounds & panels you may have to close & restart FSX for an aircraft to appear & work correctly..
Long answer to a short question..

- hope I didn't confuse you too much - gl/tc..!