It sounds like your current transmitter is an AM radio (that's what use to comes with RTR's). So you need an AM reciever. The good part, is that AM is the cheapest
But remember to check your current radio and/or manual, to be sure, if it's AM or FM.
If you have been hooked to this hobby, you might consider stepping up to a FM system, that involve both a new transmitter and reciever. Thus it's quite more expensive