If you purchase the complete compression fitting, you'll be alright. The ferrule will either be barrel shaped (cheap industrial air line type) or cone shape with a flat (style that MCP uses.) They are NOT interchangeable. You MUST use the ferrule that is designed for the rest of the compression fitting. Every kart shop in the country will have these in stock -- just be sure that you let them know which one you need, or like I said, purchase the complete compression fitting. 3/16" nylon line and barrel shaped compression fittings can be bought just about anywhere - ie, at hardware stores or most truck stops.
We stock both styles if you can't find one local to you. Give us a call.