Katy,
I don't really do much searching on the web for dance-studios any more. My Ballroom Dance directory (available free from most USABDA sanctioned events I believe) lists by state different instructors and studios in different parts of the country. Along with the Henry's Dance Hotlist (HDH) and Aria's Lets Dance pages, which are among the oldest online dance-directories if I recall, this book helps me pick out places to dance in places I visit.
I have also sometimes contacted Universities in the vicinity of the area I am visiting (also listed in HDH) and asked the students there about best places to dance where International style (or American to those it pleases) is commonly danced. The r.a.d (rec.arts.dance) newsgroup is a good place to ask such questions too, if one has access to it.
Regarding places in SF and LA, I have no doubt that there are myriads of studios there that attract international style ballroom dancers. I am not aware of any myself since I have never been to the west coast, but I am sure Jonathan or one of the others on this BBS will be able to help.
Last word. Yahoo lists Ballroom dancing under 'Recreation and Sports' and has regional listings too. I am not fully aware of how extensive this listing is, but it should complement the others on the web to provide a bigger and better picture. I hope that will help.
Sri.