Is it possible to find a webcam with large aperture?
Actually, your best bet is to use a PCIe or USB 3.0 HDMI input card, like a Blackmagic Intensity, which will take the clean HDMI output of many high quality cameras (e.g: a Nikon D600 or D800, a Canon C100 or C300, etc.) and make it available to other apps on your computer. This is what professional productions do for green screen work to ensure they're getting the full quality image out of the camera before it hits the camera's compression software.