In any shop that sells computer and/or video components. For a video-in device you can find some that connect to USB port, so you do not even have to open the computer to add a card: you just plug the device to the USB port, and then the video cable from your VCR to the device.
As you are in USA, you probably use NTSC; I am not sure that your VCR uses same plugs as in Europe. Here it is usually SCART cables, and there are cables to convert SCART to RCA or SVideo (as most of video-in devices for PC require one of these).

But is it really interesting to buy that just to grab few pictures from the video?