These pasta noodles are long and thick and are sold especially for making Pastitsio. They are called Macaronia #2 in Greece. You can find them at ethnic grocers or Greek food shops. If you need to substitute, try penne or ziti noodles.
It's important to cook them until just done, you don't want them to be too mushy. Drain the noodles in a colander and rinse them with cold water so that they cool slightly. You will be adding egg whites to them in the next step and you don't want cooked eggs.