eBay. Looking at the search results for "64DD", 235 listings show up and on the front page at least, the majority of them are 64DD units, not games or accessories like the modem or capture card. They're not rare -- at least 15,000 of them were made and sold (and who knows how many more on top of that, personally I think the number is closer to 40,000 -- that was just the number of 64DDs there were still connected to Randnet on shutdown in February 2001, but known serials exceed 35,000; I have somewhere around 11,500). You can always find a handful (or at least, I can), they're just expensive. They're kind of like the G4 Cube. It's actually much harder to find one at Yahoo Auctions in my (long and arduous) experience, and I tried for a few years before I decided to get the one on eBay that was a buy-it-now. So far, I haven't been able to do much, all my tools are stored away while I'm searching for a place to live -- I'm currently staying at my grandmother's house, and all my tools are kind of buried. I've been making progress, though, on getting to them.
For the most part, I've been talking with LuigiBlood and getting up to speed on how the software works, and of course how the hardware does. That way I'm not just wasting time and I'm more prepared for when I do have the space to take it apart. It just works like a cartridge with removable storage and a tiny IPL ROM that also has some libraries for standardized type and sound. A random fact I learned in the catching up to speed process that really stuck out to me for how weird it is is that for some reason, 64DD disks have twelve random blank sectors. I thought it was copy protection, but apparently disks read just fine without them, so it's a total mystery. My only guess was manual copy protection -- like, if you mailed the disk to Nintendo, they could notice it there, but even that's not founded on literally anything.
I don't doubt I'll be using my Macs for publishing anything I find, and talking to people; they're my daily drivers, after all. Probably see how well it handles PCB CAD, this 1080p flat panel I've somehow found myself using is kind of thrashing this 9800 Pro, but I might be able to get better results with Linux.