There are a number of companies that specialize in selling TV videos.
One good one is Shokus Video (http://www.rahul.net/shokus/index.html)
- they specialize in 1950s material. Other companies are out on the
web - see
http://dir.yahoo.com/Business_and_Economy/Companies/News_and_Media/Tel
evision/Memorabilia/Videos/ for Yahoo!'s listing of the category.
Another good resource is http://www.ultimatetv.com/ - it has a list
of programs (fairly comprehensive throughout TV history), with
external links, often to sources to buy/trade shows. If you don't
have money for buying, there's a whole cult of tape trading out on
the web - look through fan sites.
Also check out a book (how 20th century!) called _The Buyer's Guide
to Fifty Years of TV on Video_ by Sam Frank. It provides details on
buying a large variety of shows - it's a bit quirky, but really
useful.
Good luck!
-Jason Mittell
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Screen-L is sponsored by the Telecommunication & Film Dept., the
University of Alabama: http://www.tcf.ua.edu