That's the kind of info you'll find in the Typewriter Revolution, but I'll be happy to answer those as much as I can!

For the ribbons, there's no problem. These are still mass produced; I generally buy cheap Chinese ribbons and they do the trick perfectly. Others prefer high quality ribbons, which are also still produced but in lesser quantities, so they're anything but cheap. In fact the community lost a few weeks ago one of the only producers of high quality ribbons in America, but the offer still meets the demand. There's also the possibility to ink yourself your ribbons, that way you can have custom colours (or even a rainbow one)! It looks fun, that's something I want to try someday.

For the cartridges, it will heavily depends on the machine. Some brands are still produced (like the Nakajimas), so their cartridges are still produced. For others, it will depend. I avoid cartridge typewriters because of that, so I don't know much but I think the Selectric's cartridges, in spite the fact that the machines aren't produced anymore, are still produced too, so if you chose that brand it should be okay. For most others, you'll have to look on the second hand market, which is a gamble.