Ramsey has been selling a low power VHF TV transmitter kit for years.
http://cgi.ebay.com/Ramsey-TV6C-VHF-...QQcmdZViewItem
Also available from amazon.com, etc.
I built one over 10 years ago and was not very impressed with its performance.
You can also buy a "video sender" which broadcasts on a different frequency. Only rub there is that you need to buy a receiver for each of your vintage TVs, or swap the receiver around as needed. I have one of those, as well, and again, was not impressed with the range or signal quality, although it was definitely better than the Ramsey kit.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html