You can't buy them.
Most "retired" characters just get stored away for possible future use. The lesser-known characters are often used for training purposes.
But if a costume has worn out and simply can't be cleaned and reused anymore, it's destroyed.
Similar things go on with themed CM costumes. When individual pieces get worn out to the point they're no longer considered show-worthy, Disney donates what they can. Sometimes they have to do things like remove patches or other decorations, but as long as it's a pretty generic-looking piece of clothing, it'll get donated. Like jeans from Frontierland, skirts, polo shirts, khakis, button-down shirts, etc.
But if you're talking about a signature piece of clothing, like a Tower of Terror bellhop jacket, Dapper Dan striped shirt, Mansion maid's uniform, etc, those pieces are destroyed.
-Rob