See sites below for ideas.
Try Jessica McClintock in downtown San Francisco. Most of the gowns are from $170 - $400. She also has a website, but I think you can find more gowns in the real store. The online selection isnt very big. :)
Try House of Brides. And for affordable wedding jewelry and accessories, try http://www.glamforless.com.
Go to Joanne s and find a pattern - it can have very simple lines. Then find a seamstress and fabric. Have the dress made.