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What is the physical media for data communiction you intend to use (power cable, twisted pair, air, ...) ?
Which type of data coding you will be using (NRZ, Manchester, ...) ?
What kind of modem (modem -> modulator-demudolator) device you are searching.
What kind of modulation type you intend to use ?
I think you should look to OEM suppliers, they offer boards with differents modem chip (TDK, Connexant...) and TTL interfaces you can connect directly to a simple MCU. Those kind of boards are typicaly used in set top box.
I think you can't buy the chip modem alone because you will need quantity to do so.