The U.K. Access Management Federation has an extensive list of Shibboleth-protected resources that are available to U.K. federation members. Since these vendors are already Shib-enabled, they may be more likely to join InCommon and work with you as a Sponsored Partner.
http://www.ukfederation.org.uk/content/Documents/AvailableServices
Also in the U.K., JISC maintains a list of publishers currently discussing federated access management, and their known status. This may be a resource to leverage for federating with InCommon.
http://access.jiscinvolve.org/federated-access-and-publishers/
The German federation lists all of its members, including vendors (note: this is in German)
http://www.dfn.de/en/enhome/organisation/members/
SWITCH, the Swiss federation, lists its partners here:
http://www.switch.ch/aai/participants/partners.html
SURFnet, the federation in the Netherlands, lists its partners here:
http://federatie.surfnet.nl/cms/content/view/44/48/lang,en/