Welcome to the forum! This is definitely a great place for information and people here are always willing to help.
Personally, I've bought all my snakes from a local reptile store but prepare to pay more than you would online. I like to physically see and inspect my snakes before the purchase but that's just a personal thing. If you buy from a reputable sources online, you should be fine. I would also stay away from chain stores like Petco for your perchase. In MY experience these animals are usually are in worse shape.
It's perfectly normal to get a young snake. I even prefer it because you get the joy of raising your new pet.
Where in California do you live? I can suggest a great reptile shop if you're near San Fernando Valley. It's called Exotic Life Fish & Reptiles and they just recieved LA Reptile store of the year. I wish you luck in finding your new pet!
Sent from my SAMSUNG-SGH-I747 using Tapatalk