I’ve went to fumeroles and haven’t found anything special: just stuff like iron and carbon. You can go here to find specific raw materials.
https://eddb.io
Go to Bodies and type in what you want, and type your current star system into Reference System. You can arrange the list with %, so you can find landable bodies with a high percentage of what you want.
Look for the "Needle Crystals" deposits for the G4 materials. There's usually a few per site.I’ve went to fumeroles and haven’t found anything special: just stuff like iron and carbon.