I'm looking for empty LEGO boxes. I need two: one for 6389 Fire Control Center and one for 6669 Diesel Daredevil. Where could I find them?
You can buy boxes for most sets from Bricklink.com. For example, there are world-wide three sellers of the box for set 6389. Unfortunately, getting a box for such an old set will set you back $40-$50, excluding (international) shipping.
For set 6669 there are currently also 3 sellers, with prices ranging from $5 for a severely damaged box to $30 for boxes in good condition, all prices excl. shipping.
Of course there are other places to look, such as EBay or local classified sites like Craigslist in the US, but chances you find one of these boxes there are minimal. Bricklink (or its competitor BrickOwl) are your surest bet.