Mobile Phone Contract/ Official Address Card (Budapest)

I am trying to get a Mobile Phone Contract - wish to do Post-paid as it is considerably cheaper when using data. I have a residence Permit card (with no address on it) and phone companies are telling me they also need an Official Address Card. All I have is the white slip issued by immigration that has my address on it. I am told that this is not acceptable. So, what is an Official Address Card, and what does it look like, and more importantly, how do I get one? Suggestions welcome!