When I make Phillys at home I have the Dillons meat guys shave a bunch of sirloin for me. I found a marinade online that is pretty great.
2 tablespoons Dijon mustard
2 tablespoons balsamic vinegar
2 tablespoons Worcestershire sauce
1 teaspoon salt
1/2 tsp EACH garlic pwdr, onion pwdr, dried thyme, dried parsley
1/4 tsp EACH pepper, red pepper flakes
I usually use a little more vinegar and it works best if you let it sit for half a day or so in the fridge.