quoting Izambri, Duke, the Findible League:

quoting Hālian, Atetía @ Central Florida:

Your money is too beautiful for this world, how do you not have a job designing money irl

(Also I've been trying to design Hoennese coinage but drawing a blank as to obverse designs… doesn't help that I can't draw anything more complicated than simple geometric shapes myself, either vector or raster.)

Drawing fantasy money (coins and banknotes) is quite easy once you now where to fish textures and images and how to work with them. In the first case Rhetorica and bloodbath have already pointed out the basic places: Wikimedia Commons, OpenClipart... Doing extensive searchs through Google Images also helps a lot, especially for gradations, uncommon geometric shapes and other textures.

Then working them shouldn't be a big problem: I use Paint.NET, a free software, but more complex things like Gimp or Photoshop are also a possibility. To be honest, you really don't need something very professional, since playing with basic tools like gradient, magic wands and layers is practically all you need. The most important thing of all, in my opinion, is to have a clear idea of what you want to do, no matter if it's too close to real coins and banknotes or a more deviant design.

This is exactly what I recommend: find a good software that you're comfortable using and work with it. Paint.NET is my go-to, but nowadays I use a combination of Inkscape for lettering and some of the text effects and Paint.NET for the actual assembly. Have an idea of what you want to do, but also don't be afraid to try!

quoting Izambri, Duke, the Findible League:

The next series of banknotes (which I'll do some day...) will be more refined. And so on. It's all about trying to do it, failing at it, and repeating.

Bingo. I've been making banknotes for 10+ (!) years, and I've only gotten semi-good at it because of a lot of practice and a lot of designs that looked hideous. Took me about 5 years to get comfortable with the process, and I'm still learning new things.