Question: Are there other life giving sources of energy in space apart from stars? (like nebulae, radiation, etc.)? Or are all possible life giving potential sources some variation of a star?

Answer: 
There are three important sources of energy in planetary bodies (or their moons) that may be important in determining their suitability for life. All of these are evident in various bodies in the Solar System.

The first is tidal heating. When one object orbits another then there will be gradient in the gravitational force across their finite sizes. If this is combined with any orbital non-circularity or difference between rotational and orbital periods, along with some fluidity in the structure of one of the bodies, then it will be heated by frictional forces as the fluid moves in response to the changing gravitational gradient. This heating is most evident on Io in our own Solar System, the closest large moon of Jupiter, and is generally more important for objects orbiting close to more massive stars or planets.

The second is heating by radioactive decay. When planetary systems are born, they can incorporate significant amounts of radioactive material. Some of this has a short half-life and may be responsible for a short period of intense heating, but other isotopes are much longer lived. According to [this article](https://blogs.scientificamerican.com/observations/nuclear-fission-confirmed-as-source-of-more-than-half-of-earths-heat/), about 20 TW of power is generated inside the Earth from the radioactive decay of uranium, thorium and potassium. This sounds a lot, but is about a factor $10^4$ less power than we receive from the Sun.

The third source is just the "heat of formation". Gathering together the raw materials of a planet/moon and compressing it into a gravitational potential well, inevitably leads to the generation of considerable heat and it can take billions of years for this heat to leak out into space. Indeed, for the Earth, the heat of formation is probably comparable with radioactive decay in terms of supplying energy to the surface from the interior.
Stars function by nuclear fusion. There is energy that isn't released by nuclear fusion, nor from the nuclear decay of elements produced by fusion.

There is the cosmic microwave background. This is radiation, but it "cold", at about -268 \*C. There is a lot of energy, but because it is so "cold" it is difficult to use it to power anything.

There is a lot of gravitational potential energy. This can be released when things fall together. So when an asteroid hits the Earth, the energy released is not from a star. However its not a dependable source of energy for life.

There are proto-stars that are powered by gravitational energy, and there are accretion discs around neutron stars and black holes that are hot due to the gravitational energy released as matter falls into them. The hard X-rays from accretion discs isn't much good for life.

There's also a lot of potential energy in a cloud of interstellar gas that could be released if it all fused to heavier elements. However the only effective way to release this energy is to make stars: Lots of energy, not easy to get at.

In principle, all mass is energy, by E=mc². However, using this energy to power something isn't easy.

Really you are asking the wrong question. There's lots of energy. The trouble is converting it to do useful work, otherwise all this energy is about as useful as a piece of coal to a hungry person.