I used the material icon website
https://materialdesignicons.com/ to find an icon that was close to what I wanted, then downloaded the icon package (zip file). Inside the zip there are a number of folders (one per resolution) and in each folder, there are black, white and grey versions of the icon at various sizes (18dp, 24dp, 36dp etc.).
I extracted the black 24dp files into the folders named in the zip, with all folders under the <app>/objects/res folder (which now has subfolders named drawable, drawable-mdpi, drawable-hdpi etc.). Once all the files were extracted, I used Paint.net to customize the images while retaining the transparent parts.