One way I've read to fuel a theoretical interstellar space ship is using Helium 3. While not abundant on Earth, there's plenty on outer planets like Jupiter, Saturn, Uranus, and Neptune.
Using a concept like this would be great for science fiction. The article talked about a space-gas station created at these planets. But I see other possibilities. Why limit yourself to interstellar space travel? What about in-system and using the gases for other sources. A writer could turn this to corporate sci-fi with competing gas mines. This could lead to threats of in-system wars, embargos... I think I've said enough about that.