There are several slab suppliers in downtown Seattle, but Pental was the first one we went to. They have a large warehouse with (I'm guessing) several thousand slabs, all arranged by color\style:
There were several such aisles of slabs like you see in the picture above. You can walk around by yourself looking them over (being careful to dodge forklifts), or if you want to look more closely at a specific slab, you can ask the warehouse crew for help. The workers have forklift-mounted cranes so they can pick up individual slabs and place them onto heavy-duty dollies:
Once you have decided on a particular slab, you give them your credit card and the warehouse crew loads the slab onto the roof of your Honda Civic....just kidding! The way the process works, you reserve your specific slab(s) (each slab is bar-coded for identification, so there's no chance of mixup) and your fabricator will come by later and pick them up (and he'll have a good, heavy-duty truck for hauling these things around). Our fabricator (recommended by our builder Paul) is ProGranite.
The slab in the picture above is a style called "Sapphire Blue", and it and the slab immediately behind it will eventually become the countertops and island top in our kitchen. I am really disappointed in how the pictures of these slabs turned out, because they are much nicer looking in real life. We also picked out a slab of "Blue Pearl" for our master bathroom vanity top, and for some reason I don't have a good picture of that one. Ah well - next time you see them, they'll likely be getting installed. :)