If you're in Italy, just get the Kynoch stuff at Kynamco, UK. Also drawn brass, and quality stuff.
As for the rest... where to start?
Dies can be found at CH4D (US) - 1 1/4", and also at NDFS UK (small-base dies). but 1st you need to know the model you have. MKI-III will have a groove diamter of about .465, Mk IV of .470'ish. You can use .458 cast bullets, but better paper-patch them up to the diameter required. Load BP, about 85-90 grains of Fg, card wad, felt lubed wad (or beexwax plug), another card wad, then bullet.
For more info, I suggest you join us here, lots of things to tell....:
Martini forum

a friend here is looking for Bertram brass in .577/450 for a Martini Henry rifle. I have noticed that some members own such rifles so I am going to guess that the brass is availiable.

Who makes dies? Does it use normal .458 diameter bullets?

Any info is appreciated.