Apple's part number for the Mini-15/DB-15 (not VGA) adapter is 590-0831-A. They turn up on eBay from time to time. To that you can attach a standard or a switchable DB-15/VGA adapter.

You could also try direct approach to the likes of macmetex and olde-mac-milt. I know of no Mini-15/VGA, but I certainly wouldn't dismiss the possibility.

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