If available, MS is the only source for the DVC members discount for cash reservations.
If MS has no availability, CRO may have some rooms to reserve. At some times of the year, they may have DC or AP discounts available for rooms at DVC resorts.
Someone reported here this week that MS was willing to match a discounted rate from CRO for DVC members. It seems that MS is the place to start looking for cash reservations, but CRO may have something available if MS does not.
A site that keeps track of discounts available at WDW (all resorts) is http://www.mousesavers.com/disneyresorts.html
Another option other than going through MS is to rent through another owner. See the Rent/Trade board on this site.