How the voyager gets fuel for such a long time period?

No, it doesn't need any fuel. Spacecraft aren't under constant propulsion. Once they're launched on an escape velocity they just keep going forever. Voyager doesn't even have an engine. The rocket that launched it on that escape trajectory was discarded once it was no longer needed. Voyager only has tiny thrusters to change the direction it is pointed and keep its antennae pointed towards Earth. It doesn't take much to do that though. Even a short burst from something like a can of spray paint would suffice. And the Voyagers have a nuclear power source powered by plutonium, an RTG, that provides them with electricity.

Everybody seems to assume that you misunderstand the situation, but I'm not sure whether you do or not.

The Voyagers (there are 2) don't need fuel for propulsion any more. They got all the momentum they needed from rockets in the early part of their journey, and then from the gravity of the planets they passed. These 'near misses' were the principle part of the the mission, but they also gave the probes additional energy, and sent them on their long journey out of the solar system.

That doesn't mean, though, that the Voyagers don't need fuel. They do indeed, for example for their regular transmissions of data to earth, and to receive instructions from earth. This energy is obtained from the radioactive decay of plutonium, which generates heat, and form that heat, electricity is generated. It is the same basic system as Curiosity is now using to power itself around Mars.

>>Travelling such a long period need a lot of fuel <<

No it wouldn't. Why do you think so?

because space is a vacuum, there is no resistance, so once something starts moving it wont stop unless it gets caught in a planets gravitational pull, so no extra fuel is needed to keep it moving.