Look for a third-party buyer on Amazon. Although Disney itself may have discontinued the DVD, a number of licensed DVD sellers still have copies for sale. Simply type in the title of the film in question into Amazon's search engine, then click on the "Used & new" link. Prices vary wildly depending on the title and seller. A very popular title such as "Cinderella" had a big production run and thus may be much cheaper than, say, "The Scarecrow of Romney Marsh," which had a much smaller run.
Check for the title on eBay. Speculators and folks just looking to unload their collection may auction them off there, and professional third-party sellers offer them there as well. Again, prices vary by the level of demand, though it's possible to get a real bargain from a private seller eager to get rid of one.
Check conventions, flea markets and garage sales. Sellers may offer hard-to-find Disney classic DVDs at booths during science fiction and comic book conventions, and private sellers may get rid of them through more informal means like flea markets. It can be very time-intensive--you'll likely be searching through a lot of unsorted bins--but it can produce real gems available for an extremely reasonable price.