There was a research study done, which I can no longer find, which
states that around half of foster fathers are primarily motivated to
be foster fathers by financial reasons.  What is this study and where
is it published?

I don't know if this is it, but here's an article along those lines...

Accession Number
Journal Article: 1999-16100-003.

Author
Inch, Leslie J.

Title
Aspects of foster fathering. 

Source
Child & Adolescent Social Work Journal. Vol 16(5) Oct 1999, 393-412. 
Kluwer Academic/Plenum Publishers, US

Abstract
Derived from a larger study aimed at understanding the experience of
foster fathering from the perspective of the men themselves, this
study examined the attitudes of 15 foster fathers (aged 35-69 yrs)
towards being foster parents. Primary sources of data were individual
interviews with foster fathers and completion of a brief demographic
questionnaire. Study findings indicate 6 core categories forming the
context of the foster fathering experience. Of particular relevance to
the issues of adoption and permanence are the reported experience of
these foster fathers as represented in 4 of the categories: 1) On
Becoming, 2) Fatherly Functions, 3) Attachment and Loss, and 4)
Personal Connections. The decision to become a foster father involves
incentives, or motivations, that satisfy a personal interest of the
man. Foster fathers make little distinction between foster parenting
and legal parenting, indeed, they feel they have a job to do like any
father. Foster fathers believe the foster children do not have
experience with positive, healthy father-child and family
relationships. Despite advice not to get too attached, foster fathers
develop attachment feelings unless extraordinary circumstances are
present. The article concludes with implications for social work
practice. (PsycINFO Database Record (c) 2002 APA, all rights reserved)

That's not it--that article doesn't talk about financial motivations. 
Thanks though!

This may be of interest:

http://www.mainstream.com/dcyf/broker.txt