At times I've been asked how to get a memory card suitable for the Amstrad NC100/NC150/NC200, so I thought it might be useful to briefly give some general hints on that for NC owners who end up here. I'll also touch on serial file transfers, which can sometimes be an alternative of sorts, and are useful for making backups either way.
Buying the specific old sort of memory card you need for the NC100 etc. seems fairly difficult at this point, partly because I don't think they're made any more - and if you try to get a second-hand one, you'll be competing with e.g. commercial owners of expensive old machinery that takes the same kind of memory cards. So they aren't as cheap as you'd hope.
If you do want to try buying one though, I'd suggest searching for "PCMCIA SRAM" as a starting point, and making sure that the capacity is no more than 1MB or it may be unusable (so you want 256k, 512k, or 1MB basically, and it needs to be Type I and SRAM-based). With second-hand ones you also need to watch out for any damage from leaked batteries, which could have resulted in corroded contacts or worse.
If you're in the US in particular (or buying from there), it might be useful to look for cards intended for the Apple Newton, some of which I believe should work. Again, you do need to ensure that the card is SRAM-based, and suitable more generally.
Now, if you're using the normal ROM software the NC100 comes with - presumably meaning that your main concern is simply having more storage - then you might be better off just copying data to and from the NC100 using a serial link, if you don't already do that. This isn't very convenient exactly, but it isn't too bad; although serial ports on PCs aren't really a thing nowadays, USB-to-serial converters are still fairly easy to get. (You may also need a null-modem cable.) Or if you happen to have an old PC with a serial port, that's another perfectly valid option. The software side is easy enough, you only need XMODEM file transfers and there's free software that can do those. For example, I use Minicom with lrzsz on Linux (apparently a common choice on macOS too), and the open-source "ttssh2" version of Tera Term seems like a reasonable choice for Windows.
If you use ZCN, it's worth noting that you don't technically need a memory card to use it except on the NC100. On the NC200 and especially the NC150, the ramdisk support added in ZCN 1.4 allows some level of use even without a card.
Ultimately, the simplest option is the boringly sensible one, to just use an emulator like nc100em. It's not quite the same of course, but it does at least let you use a virtual 1MB memory card.