What should i look for, is there a specific name?
Traditionally it's an Italian bread shaped into a round flat loaf. You could use regular Italian bread but I'd suggest you remove a little of the innards so that you have the crisp outer shell and the inside of the sandwich isn't too doughy or soggy.
drbabs is right but I like to use ciabatta for ease in slicing
We like to use a round domed loaf for Muffuletta. It leaves room for being more extravagant with the fillings. ;) We pull the insides out of the bread and line both top and bottom with the olive salad before layering the cold cuts, cheese and veggies. It's a little harder to weight down, but it is doable.
I use hard rolls if I'm in a pinch... great for individual servings.
use a softish italian type loaf from the grocery store and pull out the insides as above. it's just a container for the vast quantities of filling.