The way Disney does things is they have a movie available ("released from the vault") for about a year. Then the movie goes "back in the vault" for some period of time, typically 2-5 years. So right now I think Cinderella, The Little Mermaid and Fantasia are "in the vault" and not available.
Your best bet is eBay and craigslist. But be careful there are a LOT of foreign imports of pirated Disney DVDs on eBay.
Beth, mom to older DD (8/01) and younger DD (10/06) and always missing Leah (4/22 - 5/1/05)