Don't know where to get slips, but I've gotten organic sweet potatoes from Trader Joe's and used a couple to grow more. I didn't cut them up to make more, because I was trying to find out if a small whole one worked, and it did!

Ive thought about using taters OG taters from traders but the reason no one ships to California is because they could have different diseases. I do not remember the specific one but if memory serves me correct it has to do with a root disease.

I know this is an old question, but I figure if anyone is searching they'll find it so I thought I'd post an answer. Organic sweet potatoes work, but if you're looking for specific heirloom varieties, Sand Hill Preservation ships slips to CA. They're in Iowa, which is permitted to ship to CA. According to CA dept. of Food and Agriculture restricted states for sweet potatoes are: AL, AR, FL, GE,
HI, LA, MS, SC, TN and TX.

That's where I got mine. Grew them for 3 years and then kinda forgot to save seed stock. It's OK, I'll start over from random organic sweet potatoes from the store next time I have garden space for sweet potatoes.