What the lawyer's secretary told you is generally accurate. I retain and supervise lawyers on a weekly basis to work on my civil litigation cases. I have had occasion to retain criminal counsel for my clients as well. In criminal matters, most criminal defense lawyers are only contacted and retained when an actual problem occurs, not prior. They generally have answering services, so if you need one after hours, merely call their office and ask the answering service to page them, or tell you where they are and ask the lawyer to meet you there. After they respond and accept you as a client, they are officially retained, and you can start writing checks to them.
Another idea would be to call a criminal defense lawyer, tell them that you would like a one-hour appointment to discuss self-defense law in your state and at the end of the meeting, ask if you can call them if you ever have a problem.
In the mean time, if you are involved in a shooting, merely smile politely and tell the responding officers that you are not being difficult, but you are declining to answer any questions until your lawyer arrives or otherwise contacts you. I suggest something like this: "that man attempted to kill me and I was forced to respond in self-defense. I cannot answer any questions until I speak with my lawyer. Please don't ask me any further questions."
In regards to calling or using a North Carolina lawyer, you can certainly do so if they are also admitted to the bar (licensed to practice) in South Carolina. In many cities/towns located on state borders, particularly the larger cities, many lawyers will be admitted to practice in both states. An example locally is Portland, Oregon. Many lawyers in Portland are admitted in both Oregon and Washington and handle cases on both sides of the Columbia River. I bet that the yellow pages ads for lawyers in Charlotte will have many listings that actually say that the lawyer is admitted in both North and South Carolina.