First and foremost go to places near you and try stuff on. See what you like on your hand, and ask for pricing. DONT BUY ANYTHING. Just look, get a feel for what you like shapewise, sizewise and stylewise. Take notes, take names and get a feel for things. I am not from San Francisco, but I've checked around the industry and got a few suggestions from friends in the area as well. A good spot to check out is Joe Escobar. They have a pretty good reputation. Gleim the Jeweler is supposed to be good but I've heard very overpriced and often pushy, Schiffmans is supposed to be good, as is Willow Glen. The ones I see most suggested are Joe Escobar and Willow Glen. You can also check out a place like Lang Antiques which I believe is also in the area, and the big ones like Tiffany and Cartier and Debeers. Just to see what sizes and styles you like.
I am an online proponent personally, I am a big fan of Good Old Gold because no one gives more data and does more testing. I think what shape stone you want makes a big deal in which vendor I typically point to, because I think GOG does most shapes well and has some branded cuts they've designed specifically for light return, but some are better for just rounds and princesses, like High Performance Diamonds (specifically those from Cut by Infinity) and also often have been recommending Brian Gavin Diamonds more recently as well (he has a new cushion style out also). But Engagement Rings Direct is amazing for Cushions.