Where can I purchase a copy of the Dateline NBC show that aired
tonight (5/7). The subject was Quixtar (a multi-level marketing
company)

Hello.

Dateline tapes (if available) and transcripts are sold by a company
called Burrelle's, telephone 1-800-777-TEXT.

From the Dateline web site:

"Transcripts for NBC News Programs are available for purchase through
the Burrell Transcript Service by calling 1-800-777-TEXT. Only
selected segments of our news magazines are available on VHS
cassette."
source: 
Dateline: About Us
http://msnbc.msn.com/id/3360263/

Burrelle's/Luce Transcripts & Tapes - web site
http://www.burrellesluce.com/tt/ttothers.html

Dateline actually has quite a bit of information about tonight's
Quixtar segment on its web site:
See:"In pursuit of the almighty dollar
Dateline investigation: Inside story of business that attracts people
with promise of easy money"
http://msnbc.msn.com/id/4375477/
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search strategy:
browsed nbc.com

I hope this helps. Good luck!

Thanks.