Where to buy 400K and 800K Floppy Disks

But that's like formatting 1.4MB disks as 800K disks and I know you're not supposed to do that.
You're right. You're not supposed to format 1.4MB disks as 800K disks or format 800K disks as 1.4MB disks. You're not even supposed format 400K disks as 800K disk. But you CAN format 800K disks as 400K disks. (If you really want to know why this is true, read on.)

In order to understand why this is possible, you need to know about the different types of disks and drives:
An 800K disk is a double sided/double density (DS/DD) disk.
A 1.4 MB disk is a double sided/high density (DS/HD) disk.
When a DS/HD disk is made, the disk (and its magnetic oxide) is only certified and tested for data at 1.4MB. While it seems possible that there would be no problem formatting those disks at 800K, the oxide formulas are different. The same is true for 800K disks: the disk (magnetic oxide) is only certified and tested for data at 800K.

But here's the catch: the DS/DD disks are certified for 800K, which means 400K PER SIDE. Remember, a 400K disk is a single sided/double density disk (SS/DD) and the 400K drive only has one read/write head; it only read/writes to ONE SIDE OF THE DISK. All other Mac drives (800K, superdrive) have two read/write heads and read/write to both sides of the disk.

The important part is this: it's the disks DENSITY that is important regarding the compatability of the disk in a particular drive and not the disks storage capacity. In other words, since a 400K disk drive is only read/writing to ONE side of the disk and BOTH sides of 800K disks are certified for read/writing, you can safely format 800K disks to 400K. Of course, the reverse isn't true: you shouldn't format a 400K disk to 800K because only one side is certified and you might lose data on the "uncertified" side.

QED