I know how to create awesome looking, easy to use surveys with
http://www.surveymonkey.com but I want to know what is the cheapest
independent service which will find me 5,000 "qualified" users to take
my survey? In this case, I just want American head of families which
cut across all demographics.

Identifing heads of households is difficult on the internet, and if
they are identified from people who bought or sold houses, then they
are not an across the board demographic. Did you want them qualified
in any other way besides head of household?
 Even if you pay people to answer your survey, less than 10% will
respond when you send them an Email. I delete 60% of my Email without
determining what the message is. The percentage is a bit higher, I
think, for post office delivered mail.
 If you give us more details someone may have some good sugestions.  
Neil

I do not have any other criteria than the one mentioned: American head
of family households, cutting across all demographics (income,
education, race, religion, etc).

I would never sponsor a survey by spam. (See
http://www.SpamSubtract.com which is software I designed.) I am
looking to pay a professional research group (or university) to simply
find a way to get 5,000 qualified users to take my survey, and I'd
like to know the most cost effective way of doing this.

I realize I am going to probably have to accept a slight bias in the
selected set (e.g., testing only people who are on the net) for cost
reasons.

Ok, my survey has to do with the role of television in their families,
and also on criteria the television buyer in that family use to select
TVs. So maybe I could have my survey attached at the end of an
ecommerce purchase (complete this 1 minute survey for a chance to win
a new TV?) or maybe I could buy an ad on a TV website, although I'd
worry a little about those that would answer such an ad possibly
having a different bias than the general public?