Imagine a type of particle accumulator and aggregator - call it cosmic flypaper. If you're going say 100 km/s, things seemingly insignificant and rare you may be able to collect a lot of pretty quickly.
There's also nebulae. But that's only going to be useful when you're actually in a nebula, which isn't most of the time.
IIRC, a Bussard Ramjet is at least potentially viable at the typical density in the galaxy, but unfortunately for any use by humans to get away from our solar system, or local region of space (to a distance, IIRC, on the order of 1000 light years in any direction) is a pocket of relatively low density.