I'm experimenting with a sci-fi setting where human-like mechanical androids are the dominant "lifeform." Though the setting is based on science, androids live in tribal societies. For the most part, these societies are devoid from other pierces of technology - there are no high-tech vehicles, weapons, or power stations. As such, I need a way for androids to power themselves without the use of batteries or power stations like hydroelectric or nuclear power plants.
Futurama has a good suggestion:
Alcohol fuel cells are what you need. Grow suitable crops to provide carbohydrate feedstocks, then ferment and distill to produce nice, relatively purely fuel.
Distillation can probably be done with bronze-age technology, certainly possible (but perhaps only in a limited way) with iron-age level knowledge and was clearly done by medieval societies so you have plenty of leeway in the sorts of society your androids are emulating. The resulting fuel is quite energy-dense, relatively stable and storeable, and the fuel cells can certainly be made to be powerful enough to make a human-sized thing capable of doing human-equivalent acts of physical exertion. Internal fuel tanks can tide the androids over between meals.
The nice thing is that this kind of power source makes good sense for a fairly high-tech society of the sort that would have produced your androids in the first place, removing the need for additional handwaving in the backstory.
To simplify it to an uncomfortable degree: you need material to grow or maintain the androids and energy for movement.
There are differences between humans and robots in how they gather and process both of these. For robots it's generally simple. Your parts are refined and placed/replaced according to your needs. Energy can come in the form of electricity or chemical bonds, like diesel, among other methods. A human is more complex. We eat food that is broken down and processed, stored and used for both (in addition to oxygen).
Ultimately this means that food can be a power source for any movement, as well as building material. I don't know how your androids function, but a few meals a day could power them and potentially help them grow/maintain themselves. You can enlarge/change their diet, so they can process the required materials for their android forms. You can do this in many ways you want. Examples can be it is transported and used much like blood, or maybe they have a small advanced 3D printer inside them to create the parts required, which can be used by the android or someone else for construction/maintenance.
If you make some of them solar-powered, you can add a twist: droids which are active only by day. They can be made to hibernate for an indefinite amount of time by being pushed into a hole or cave, and going under foliage makes them slow.
There is a kind of power source called Radioisotope thermoelectric generator, or RTG for short. It is used in satellites and space probes - all the five probes currently leaving the solar system are powered by it.
RTG's extract energy from the decay of certain radioactive elements. In the wiki article it mentions some isotopes with half lives over 400-years. It may be that long ago humanity left a lot of RTG powered droids around, and maybe also many extra RTG's for spare parts that the droids can use to build new ones. Over centuries though this would stop working, so the droids might be in a quest to find new power sources soon.
Your androids are living without other tech; you explicitly rule out power stations etc. They are living in tribes like humans.
They could power themselves with chemical energy. We do that. We eat reduced carbon molecules that can be oxidized to CO2 to release energy and then couple that energy release to charging up ATP which is biologically available energy.
It is not very androidy to eat blueberries and pork. You could obtain the chemical energy through a different oxidation reaction. The oxidation of metallic iron (or aluminum or titanium) also releases energy. Lots of bacteria oxidize reduced iron for their energy. Your androids could eat reduced metals and oxidize them within their bodies, powering their internal functions.
I am not sure about the greater picture for your androids living in tribes with no tech but it seems like at one time they were part of something more technologically advanced, and have fallen to their current state. Maybe there are ruined devices around ed to be, and maybe they themselves remember.
Long ago these androids had proper fuel. Now they break little pieces off of the old machines and eat them. The androids are cannibalizing their past. It might help your story.
It is worth pointing out that the androids themselves have reduced metal titanium / aluminum skeletons. In isolated groups, some individuals might have skeletal components now replaced by wood.
Hydrogen Fuel Cells
Basically, hydrogen fuel cells produce power constantly as long as there is oxygen and hydrogen to sustain it. Oxygen can just be extracted from the atmosphere, while the hydrogen can be purified from ingested H2O. As a result, as long as they have air to breath and water to drink, these robots can sustain themselves.