I get mine from various sources, often through an online auction. They come in quite a variety of formats, some better than others. But the best part is that each one contains a free, on-topic, vintage bicycle! I make sure to buy at least one shipping carton this way every time I even *think* about selling a bike - that way I always have plenty of boxes on hand. Theses boxes usually run in the $100 to $500 range, with the occasional outlier in the $50 or $1000 category.
Portola Valley, California USA