https://www.google.co.uk/search?q=Sources+Sought :

A Sources Sought is not an actual bid or proposal solicitation; instead, it's a solicitation of interest. Think of a Sources Sought as market research being conducted by a government agency to determine what the capabilities and interests of the marketplace are.

http://www.colorado.edu/engineering/civil/db/DBS/glossary.cgi?word=Solicitation+of+Interest :

Solicitation of Interest

A request from a prospective client for the expression of interest by construction professionals in participation in the client's project.