I don't know if there are any companies that still sell them without the huge price tag but you can probably find an old LPB coupler box for sale from someone.
We've discussed DIY couplers here before but I personally wouldn't go messing with my mains panel. I suppose it's possible to couple to a power cord and plug in near where the main power comes into a house and use something like this.. darkliferadio.proboards.com/thread/991/carrier-current-matching-circuit
It's possible to inject the signal into the neutral line too in some house setups and get good range along with a safer setup by not using the hot power line. This method is very specific to how your house and outside pole is wired and where the ground rods are at.
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