It could also be a "marketing consultancy firm" o "marketing agency", but it depends on your specific context. Hope this helps

Realty Times - Real Estate News and Advice
... sales are the "in-house" sales and marketing staff (hired by the builder to work directly for their firm) or engaging the services of a marketing firm, ...
realtytimes.com/rtcpages/19990514_mrktingfirms.htm - 34k - Cached - Similar pages
MAPC Careers : : : Marketing
Either type of organization has the option of hiring the services of a marketing firm to increase the sale of its products or services. ...
www.clemson.edu/caah/mapc/market.htm - 25k - Cached - Similar pages
[DOC]
HIRSHHORN MUSEUM AND SCULPTURE GARDEN
File Format: Microsoft Word - View as HTML
Finalize POB branding campaign, with NPG and the services of a marketing firm, in order to target the ultimate goal of 2 million visitors ...
www.si.edu/about/budget/2005/28-SAAM.doc - Similar pages
[DOC]
HIRSHHORN MUSEUM AND SCULPTURE GARDEN
File Format: Microsoft Word - View as HTML
... Successful implementation of POB branding campaign, with NPG and the services of a marketing firm, in order to target the ultimate goal of two million ...
www.si.edu/about/budget/2006/2006-21-SAAMnarrative.doc - Similar pages