Assuming that there are no compatible cable boxes anywhere in this country, which is highly doubtful, you can certainly purchase one on your own. The caveat being that your cable company will need to enter the decoder into their system and activate its use. You'll need to speak with your cable company.
Be prepared to watch their heads explode when you do. You will be the first person ever to ask about this, they will argue up and down how all of their boxes are white so any box that is coloured black cannot possibly work. They will have no idea how to press F6 to manually enter a decoder ID into the system and they will not be able to figure out how to stop billing you the rental charge for the cable box you now own outright.
The chances are really good that a decoder sourced from a local competitor will work just fine if you can overcome the technical hurdles presented by the counter people whose basic concept of technical support is to show you that their box in the office works just fine therefore so does yours.
"We won't have any decoders for six months", makes no sense, can't possibly be true for a company that makes it's money delivering a signal to customers and smacks of, "I'm late for dinner and I can't be bothered to deal with your particular issue."