Yeah, Alice, it is important to find out which plants are potentially harmful to the ecology of your particular state or region.   Sometimes they can become invasive species.   Sometimes it doesn't matter - e.g., if the plant is a type that already grows in that area (e.g., roses, tomatoes). Very few of the plants in southern California are native, but not all of them are problematic.   

Good suggestion about checking with your state's dept. of agriculture.   (As for plant diseases, they can pose a problem even within a state or small geographic region. It's interesting that California doesn't allow people to bring in fruits or vegetables from other states by car but if you fly in, they don't check!)

There are many companies that ship seeds and plants all over the country (and beyond), but they probably have to deal with some restrictions on what can be sent where.