Sorry to say, there is no such device. Anything sending transmissions that far needs a bigger power source than, say, a comlink. However, you could do a couple things. One, get a regular old comlink message to someone who has access to a subspace or hyperwave transceiver, someone within about 50 klicks of your character. Two, have the character themselves get to a more powerful transmitter, like you would find on a ship or maybe a private building.
It’s also important to note that the Empire shut down large portions of the HoloNet (the network used to carry hyperwave transmissions), causing the Rebellion a lot of trouble when it came to communicating with each other. It also didn’t cover a lot of the Outer Rim, but there were secret channels used by the rebels. Subspace transmissions were not routed through the HoloNet, but they did take significantly longer to reach their destination, and they were easier to intercept. Think about which would work better for your story.
I hope that helps!
~ Jacen
In the same vein, if you have someone using a comlink while on a Star Destroyer, could it somehow piggyback on the Destroyer’s transmitter to get enough power to reach someone on a different planet?
I suppose it could be possible, if they managed to get a feed between the comlink and the Destroyer’s subspace transmitter. They would likely need a direct, physical connection, or at least some sort of tampering with the transmitter itself.
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