Hey man!
You'll love Seattle. It's a beautiful city, with (mostly) intelligent people. Buying pot on the street will be sketchy. I spent some time there last year. I slept on the street. Homeless kids pulled knives on me. I smoked a spliff with a crackhead who didn't know what month it was, let alone the time of day it was (actually it was 3 am). It was a grand time.
First of all, fuck the haters. You probably won't find anyone on this message board willing to sell you marijuana. If you are looking for weed, be careful. Buying it on the streets is kind of sketchy. Don't hand over your money to somebody you might have a bad feeling about, and don't ever hand over your money unless you can actually inspect the weed. It is a buyer's market, for the most part. If you don't like the drug dealer's product, tell him "no thanks." I have bought weed on the streets in Portland (a very similar city) and the guy wanted 20 dollars. I handed the bag back to him, and said "it looks more like a 10." As I started to walk away, he was desperate enough to sell the pot and told me he'd sell it to me for 10. Seattle is a little rougher than Portland, so be careful.
If you are traveling by amtrak train, you should be OK just smuggling a little on your person. The trip from Cali isn't that long, and they don't usually inspect people like they do on an airline. I've taken a 1/4 oz on the Amtrak from portland to seattle, and they didn't even give a shit.
If you are truly desperate enough, you can smuggle it on an airplane. Put it in your ass. Swallow it. Or make a tincture out of cannabis. Dissolve some keif in 151 rum. Most people wouldn't even know what to make of it since cannabis tinctures are fairly uncommon. It might be an option.
If you're driving, put the ganja in your trunk. Make sure your tags are up to date, and your plates aren't expired, and make sure your car is in good repair so the cops can't stop you for some stupid reason and say, "oh by the way, give me the pot you fucking CRIMINAL"
If you want to buy pot in Seattle, you're probably better off walking around through the main streets downtown. Walk around the waterfront area. Most likely, somebody will solicit you for weed. If they seem reliable, you can probably buy it off the street. It might be skimpy, pricey, but at least you'll get stoned. Pioneer Square might be a good place to buy your weed, but be low-key because police like to park their cars around that area and keep the area secure. They normally won't bother someone smoking weed--i have smoked joints within several feet of police cars, and they are mostly looking for crack/meth etc.
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Jeez dude why are you going to Seattle? You'll be so close to BC that you should just take a little train ride up there and bring back some medication. It might be easier to smuggle some pot over the border on the ferry than it would be to smuggle it on an airplane!