It’s been many years since 5.25-inch floppy disks suitable for Commodore, Apple, Atari, and other vintage 8-bit computers (not to mention IBM PCs and PC/XTs) have been something you can buy at the store down the street. I found some 360K DS/DD disks on Amazon, but they aren’t available in huge quantities.
But finding them online is your best bet today. Sealed, unused boxes turn up from time to time in dusty corners of offices and as part of people’s estates, and they eventually find their way online. What you need to look for are 5.25-inch double-density disks. Double-sided disks are fine in Commodore, Atari, and Apple computers. Often they’ll be preformatted for IBM computers, but that’s fine. You can easily reformat them for use in other machines.
If you want to use both sides in a single-sided Commodore, Atari, or Apple disk drive, a practice called making “flippy” disks, here’s how to do it. You’ll have to cut a notch in the other side of the disk so you can flip it over. Dedicated punches for the purpose used to be commonplace, but today they’re even harder to find than the disks. You can use a nibbler, intended for cutting squares in thin plastic or sheet metal, to cut the hole. Two bites from the nibbler makes a hole of the perfect size. Flip a disk over, line up another disk over it, lightly trace the location of the hole with a pencil, then position the nibbler and squeeze once to cut half the hole, then push the nibbler into the cut and squeeze again to cut the other half.
The practice has a downside. It causes disk drives to get dirty a lot faster–disks have a cleaning surface that sweeps dirt into a corner, but when you spin the disk in the opposite direction, it causes that dirt to distribute itself across the surface of the disk again. That said, in spite of the repeated warnings in user manuals and magazines of the day, just about everyone I knew did it way back when. Or, at least the people who owned Commodore and Atari computers did. I understand the practice was less common among Apple owners.
Contrary to what Wikipedia says (or at least implies), this was a common practice for a long time. IBM owners had no reason to do it, because their drives used both sides of the disk anyway. But single-sided drives remained standard on 8-bit computers for the entirety of the 1980s. Even after Commodore introduced its double-sided 1571 drive, some of the 1571 owners I knew continued to make flippies, even though it wasn’t necessary. Commodore owners got into that habit with their 1541 drives, and continued after they upgraded.
And considering a box of 10 disks is worth more than the disk drive you use them in, I guess dirtying up the drive is less of a concern now than it was then. Most vintage computer enthusiasts aren’t too shy about opening the case to clean a dirty drive head. Back when disk drives cost $200-$300, most end users were shy about opening them up.