I heard that the Voyager satellites used a radioactive source to power the computer and sensor systems. How much power can this technique produce nowadays? Could it power, say, a fighting robot? What materials could be used? Is it legal for a civilian to get that much material?
The Multi-Mission Radioisotope Thermoelectric Generators (MMRTG) used by NASA are probably unfeasible for the purposes you are thinking of, although they are highly useful for space exploration. Multiple spacecraft have used these, including both Voyager 1 and Voyager 2, New Horizons, Curiosity, and the new Perseverance. This is due to their ability to produce very stable amounts of energy for a long time. Unlike with solar panels, you also don't have to worry about how far from the sun you'll get.
According to this NASA fact sheet, the MMRTGs on Curiosity and Perseverance are able to supply 110 Watts of power. In comparison, an average desktop computer can use up to 300 Watts of power during peak operation. I highly doubt it could power a fighting robot. The reason these are used on space missions is because they provide very stable and constant power.
As for whether or not a civilian could get their hands on some, that would be a big no, for several reasons. While the Plutonium-238 used in them is not generally weapons-grade, it is still highly dangerous radioactive material.
It is also extremely hard to come by. It is difficult to produce, and the United States only produces a relatively small amount. The US recently restarted production of Pu-238 in 2013, but are only able to produce a few hundred grams every year. They hope to be producing at least a pound per year by 2023. However, each MMRTG requires 10.6 pounds of plutonium dioxide.
Use of plutonium is regulated by the United States Nuclear Regulatory Commission, and I can't really see them giving 10 pounds of Pu-238 to a civilian when it's so hard to come by already.