I want to do some cheese steak at home. I was wondering what kind of meat should I buy.
There is no single cut of meat that is universally used in Philadelphia. Top round is common, and it may actually be the most traditional given the sandwich's Italian origins (top round is what is used in braciole and Italian beef sandwiches). It is becoming more and more common to use rib-eye, though, which is what is used in some of the most popular Philadelphian establishments (e.g., John's Roast Pork and Tony Luke's), which makes sense given its higher fat content.
As lazoDev mentioned, you should be able to get your butcher to slice the meat into thin (~1mm) pieces. If that is not possible, though, you can put an entire rib-eye roast into the freezer for a half hour or so. This will harden the meat without actually freezing it solid, and will make thinly slicing by hand much easier.
As a person who actually sells Philly steak meat to many of the national chain accounts and most of the cheesesteak accounts around the Philly area, the raw material used to make a true Philly steak is knuckle meat or wedge meat. These are both cuts of meat you will not find in your local grocery store.
Thousands of cheesesteak accounts claim they use only ribeye, but very few can afford the high cost of ribeye and having ribeye advertised on their menu but serving something in place of ribeye is actually a crime, but who is going to turn them in? Chances are nobody is going to fight that legal battle and that is why they get away with breaking truth in menu laws.