They are only useful as Rock-It Launcher ammunition. They are somewhat rare except for random placement in containers or on creature bodies.
- Merchants that specialize in miscellaneous items may have on for sale.
- Feral ghoul (Fallout 3)|Feral ghouls]] and centaurs have a chance to carry junk items, including harmonicas.
- Trogs also carry junk items occasionally, including harmonicas.
- Occasionally, swampfolk carry harmonicas.