I've purchased refrigerated tapioca pearls at ranch 99. They still need to be cooked, instructions on the clear plastic tub with white lid. They were located in the refrig. section with the noodles on the top shelf, but I don't know if each 99 has it in the same section. I tried using the dried ones, but did not know to cook them right.
They should have them at the big Asian markets. I definitely saw the jarred (i.e. already cooked ones) at Manila Oriental Market the other day...they had several types, also had sago. I've never tried this before, but the syrups they were in looked somewhat unappealing. I would guess that this could be found at markets in Chinatown, Richmond, Oakland, Daly City, etc. etc.
I didn't notice if they sell the dry tapioca balls that you make yourself at Manila Oriental Market. If you do end up making it at home, keep in mind that it's kind of hard and takes quite a while. But you should be able to find some good instructions. I made boba at home only once and they got pretty gloopy on the outside before they had even cooked on the inside....definitely important to follow directions and have patience.