For Miles: Pick up your standard Spider-Man costume from the costume store and then pair it with a loose-fitting hoodie with a red hood.
For Gwen: Wear a tight white shirt under a full-length black leotard and then throw a white hoodie over it. If you're feeling really adventurous, you could chop your hair into an asymmetrical lob like Gwen (but you could always use a wig if you're not ready to take the plunge).