I've just been repurposing Amazon boxes. It helps when you buy as much junk from Amazon as I do. Try checking CL for people getting rid of boxes, often advertised by people who are moving or have recently moved. Also hit up liquor stores, they have some awesome boxes that they're often happy to part with.
the USPS priority 6x12x12 works is a very snug box for many 12 inch speakers. An inch or so of padding and another box slightly larger is a great combination. Office depot also has a 12x12x6 and has a 16x16x10 that works well for an outer box.