Finding green coffee beans is not as hard as you might think. Coffee shops that roast their own are usually happy to sell to you, most will do so at half the cost of roasted beans. You can also find them at some health food stores (Rainbow in SF) and online. The going rate for good quality specialty green beans is about $4-7/pound, though it will be lower if you buy very large quantities and a lot higher if you're interested in super premium or usual coffees (cup of excellence, Kona, Jamaican Blue Mountain, or even Kopi Luwak).
Two great online suppliers are 7 Bridges Cooperative
, which sells organic coffee roasting AND home brewing supplies, so you can get your fix of two awesome things in one place and Sweet Maria's
, which has a virtual encyclopedia of coffee and home roasting information, many of the worlds very best coffees, often sourced directly from farmers.
An alternative source to the above vendors are online coffee bean buyers clubs, two are Green Coffee Co-op
and Green Coffee Buying Club
. I've had positive experiences with both!