Autism support dog or my own dog

Hi guys, does anyone know how to go about getting assistant dog for autism? 

I struggle being independent and going outside and think a dog would really help that. Do I have to get a dog from a charity or can I train my own dog and that legally be my assistant dog and be allowed everywhere with me. 

This is all very new to me, but I think a dog would really help my quality of life, so thought I’d see if anyone else here knows anything etc  

Thanks