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: 
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.
The answer by mentioned two internal sources of energy in planets and moons, etc.:

one) Tidal heating. In some situations tidal heating hasno significant effect, in ohters it can make a cold moon warm enough for life, in others it could cause a runaway greenhouse effect.

Two) Energy produced by the decay of radioactive isotropes.

But there is at least one other sourceo of internal energy in planets and moons, etc.

Three) The left over heat from the formation of the world as grains of dust, molecules of gas, and larger objects fell thousands or millions of miles toward each other and eventually impacted with great force.