I am looking for an agency (or another way) to hire door to door sales
people who will work on a commission only basis. Anyone know an agency
that specialises in this service?
NOTE - this question is relevant for the UK market

Depending on where you are looking there are private firms.  For
example I worked as a Door-to-Door Sales person for a commission only
sale for the Los Angeles Times thru a company called JM Marketing. 
They are a Contractor for the Los Angeles Times.  I am sure there are
companies that would be willing to sell your product at the door.  We
also sold Entertainment books and gifts.

-Good Luck.

Either Jobcentre Plus will do this for free although you would have to
undertake the interviewing and selection process yourself, or pretty
much any employment agency will undertake the legwork for you at a
price.

It really depends on the sort of people you are looking to hire- the
jobcentre will just send everyone who asks on for you, employment
agencies will screen and only pass on those applicants who meet your
requirements on paper.

The difficult part is actually drafting the contracts in such a way
that the Inland Revenue will not seek to challenge the employment
status of your team. If they did mount such a successful challenge you
might find yourself considerably out of pocket.

Thanks Beastlyboy,
I heard that there are some agencies in London that arrange for 15-20
staff to "descend" on a neighbourhood and saturate it with door to
door marketing.
This will be the ideal situation for me as I will pay the agency directly. 
I certainly wouldn't want to hire anyone myself, just pay an agency per sales made.