You just tell your dad that there's a whole mess of people here on the forum with garter snakes that are far from wild. Captive bred for generations and I'm sure somebody here would be willing to send you one of those. However, also point out that if you get a local "wild" garter snake it would be educational and give a chance to gain some experience with keeping them. If something goes wrong and the snake doesn't do well, or you otherwise can no longer care for it, you can always release a wild local snake.

All pet dogs are technically "wild" too. Canis Lupus (grey wolf) is the same species as all pet dogs, even chihuahua's!. Pet dogs are simply artificially selected for certain physical traits/colors/genes/attitude, etc. Same goes for captive bred garters! Just like a chihuahua couldn't survive on it's own in the wild, many captive bred garters, especially albinos and snows, stand very little chance of surviving without human care and protection. Therefore according to your dads ideas, they would no longer be wild snakes.

That's a bummer about not getting a garter snake though. Heck, I don't even have one right now. It's been a few years and I joined the forum because I figured it was about time I got over my pair of Oregon Red-spotted. They died a few years ago of old age and it broke my heart. I figured it's time to move on and get some more this coming spring (or sooner)

Keep at it. When your dad is in a good mood, especially when he's giving you a "pat on the back" slip in a few arguments why a garter snake would make a great pet.