I am looking for an 802.11b or 802.11g video transmitter that has
functionality similar to wi-fi cameras made by people like Linksys and
D-Link. The trick is that I don't want the camera itself. I want to be
able to plug in an NTSC video source to transmit -- from any standard
device I choose. It is important that this device be accessible to a
Wi-Fi network. It would also be nice to find something with in price
range similar to the cameras i've mentioned.

To meet your required setup you will need the following hardware
(similar products may be used)

1.  You will need a transmitter.  I suggest the Ovislink WMU-9000VPN. 
This is one of a wireless routers available with a USB port to which
you can connect USB devices including Webcams.

http://www.synchrotech.com/product-usb/video_01.html

2.  You will need to have a way to now convert the analog NTSC signal
from your camera to USB.  To achieve this try any of these products:

http://www.synchrotech.com/product-usb/video_01.html

or

http://www.usbgear.net/USB_Video_capture.html