There's nothing "ready made" out there to get the proper shape. A face shell isn't about just having something under the mask, it's about having something under the mask to keep a consistent shape to the face. So you'll need to find a way to make one yourself.
My suggestion is to get a plain white face mask like these:
Amazon.com: Plastic White Full Masks: Arts, Crafts & Sewing
The using some Magic Sculpt or Apoxie Sculpt (Same product, different brand names for two-part clay that hardens to the the strength of wood) shape it to the proper Spidey-profile. Just look at pictures of the real face shell and match the very slight curve to the nose and the overall gentle sloping shape.
The sand it smooth, seal it, and wear it!
...just make sure to cut Spidey-eye-shaped holes in the face before applying your Apoxie Sculpt so you have plenty of visibility.