You're kidding, right?  I don't mean to be rude here... but while DNN is a great portal framework and there are lots of modules and resources for sale... most companies who invest to develop their critical business applications on top of it won't sell them.  It costs them more in competitive advantage than they could ever hope to recoup... and likely their implementation would be customized to their business process and not fit your client anyway.

You can certainly get and or use any number of modules for more generic types of functions.  But in-depth vertical market applications are not likely to be available.  And I would be wary of those that are... probably developed by folks not actively involved in the business... and so likely to be questionable in terms of fulfilling requirements.

Good luck... Sounds like a great custom module development opportunity.

Scott Willhite, Co-Founder DNN