I work at a thrift store for building materials. However, I'm in DC, so I don't know what's out in San Francisco...but I can help you find them! Each of the following websites have state-by-state directories for salvage yards, home improvement thrift stores, and reuse centers:

http://redo.org/SearchRedo.aspx

http://www.habitat.org/env/restores.aspx

http://bmra.org/listings/directory-map (Note: this is the site for the Building Materials Reuse Association, the trade group for the reuse industry. So while this directory lists stores, it also lists sawmills that mill reclaimed wood, and deconstruction companies that take homes apart carefully as an alternative to bulldozing).

Good luck!
Ruthie

Building REsources, 701 Amador Street is exactly what you're looking for.

I checked and found that there are several Freecycle groups in your area. If you're not familiar with that, go to www.freecycle.org, then enter your area. Around here, items go pretty fast, but you can post what you're looking for--old fencing, pallets, etc., and you might hit it rich. I was able to find a whole kitchen's worth of solid wood cabinet doors. Also, reading the "wanted" posts is a great way to remember things I have that I'm not using and don't mind getting rid of.

A salvage list for Bay area
http://www.ohmegasalvage.com/Showroom%20Pages/salvage_list.htm

See Heritage Salvage in Petaluma on list

All of the above, but I built shelves and other things besides from driftwood. It does entail regular walks along a suitable beach!
Some images here: http://www.apartmenttherapy.com/sf/house-tours/tricia-and-stefans-seaside-havenhouse-tour-103848
It helps to have access to a truck.

i second lori, above. my roommate and i just purchased some outstanding salvaged wood from heritage salvage in petaluma. the owner, bug, is amazingly helpful. they have a great selection, from newer planks to old barn wood to milled burls with live edges. they will cut anything down to size for you, and their prices are very reasonable. in my experience, the salvage yards within the city tend to have more interesting, partially finished salvage items like doors, windows, and fittings, but not a great selection of raw materials. if you just want wood, i would definitely suggest hitting heritage first. good luck!

http://www.heritagesalvage.com/

Urban Ore in Berkeley!

Earthsource Forest Products in West Oakland (formerly known as PALS Lumber)
http://www.earthsourcewood.com/fscbuilding.aspx
An amazing resource.

Driftwood/Whole House in East Palo Alto (located right near Ikea)

Builders Resources of 3rd. and Ceasar chavez right pass the bridge on your way to hunters point. Good Finds.

http://www.scrap-sf.org/

I second the Urban Ore comment.

The ReUse place on San Leandro in Oakland. Great prices, really nice staff. I'm there every other week for projects in our Fruitvale loft. A bonus is it's on the same lot as a Habitat 4 Humanity store (over priced in general, but great for paint) and a St.Vincents (for furnishings).

I third Urban Ore. More processed wood than raw (lots of orphaned doors), but it's so huge, you're bound to find what you're looking for eventually. They also sometimes negotiate prices, so don't be afraid to ask.

Fourth on Urban Ore. I've found tons of wood planks there that would have been great for shelves.