Yahoo! Enterprise Services is a consultancy arm within Yahoo!, almost completely dedicated to helping clients plan and execute e-portal implementations (until recently, exclusively PortalBuilder implementations, although they now offer other e-portal solutions). They also have relationships with system integrators such as Cap Gemini, Ernst & Young, Accenture, and Tata Infotech (a company I personally can highly recommend), who aid in such implementations.
IBM, of course, offers a full range of consultancy and support services for their e-portal solutions and any work involved in integration with other applications. Similarly, Sybase, Sun, Microsoft, and Oracle have consultancy arms that will be only too happy to help or to take on full responsibility for implementations of their respective e-portals. BEA and Plumtree have their own consultants who can help with implementation.