There's an enterprise car rental in Anacortes where you can return your a car if you want to go that route. (The real adavantage to a rental car is that you forget stuff)
Otherwise, there are a number of routes you can take from seattle. There's an airport closer than SEA-TAC but its very expensive.
Otherpeople follow the Columbia River up and join the route in Kettle Falls, WA. Less hills that way. . . .
There's one shop in Anacortes, it should be listed on the ACA route.
Another route to think abouyt is a variation of the one I'm on. I left the N. Tier at Winona, MN and cut across WI on a very cool rail trail (3 tunnels, great scenery, no hills in a rather hilly area). Then i cut down from Reedsburg to Madison, WI. There you could take the trail to Milwakee and there there's apparently a fast ferry there. I'd research that too. (If you want more specific routing just ask).
I'm cutting south through beloit. I'm in De Kalb atm and will rejoin the N Tier near the Indiana border. Doesn't save any time, but very scenic and WI is the best state toward cyclists hands down.
Most important advice: In Washington near the beginning always make reservations, hotel or campground, same with Minnesota. Also motels in Harve, MT.
There's also a fantastic campground on a lake partway up Wacoundia pass coming from the West. its on the map and 6 miles off route. Worth every miles, it on this lake that's just awesome. I stayed in the private campground which is owned by nice people. Grocery store is not well stocked, reswturant's good for breakfast.
Sherman pass is easier than it looks.
The owner of the bike shop in Clark's Fork is very cool.
After Clark Fork, ACA says you should leave the state hwy an dgo on the other side of the river. Don't do this. Your wheel's will thank you.
The first B&B in Montana costs $90 and would run $200+ anywhere else in the US. If you ahve the money. . . . Also, the perfect distance from Sandpoint for the day.
The bike shop in Whitefish is awesome. Its also your last bike shop til Havre, which will be your last shop til Minot, ND.
Take the Marias Pass alternate, Going the Sun is beautiful and the most dangerious road I've seen in a first world country for biking. If you have time, rent a car.
The Issac Walton Inn in Essex is very expensive, but very comfortable.
For the love of God do not eat/sleep at the Snow Slip Inn near Essex going up to Marias Pass.
Don't drink the water in East Glacier, they're under a boil water order.
Browning, MT sucks
There's a guy in Shelby, MT who will give you a room for the night. He hangs out in a bar on Main Street. Just hang out with your touring bike
(Same I'm told with the Mexican resturant in East Glacier and somewhere in Libby, MT)
If you have the legs, stop in Chinook instead of Havre. I just liked the town.
When ACA tells you to get on the BIA hwy in MT, follow it. It avoids some basically permanent road construction.
Malta and Glasgow, MT are full due to BN work. Call ahead to a motel if you're not camping.
There is no real bike shop in Williston, ND.
Don't ride the route between Williston and New Town because of heavy RV traffic. Bring DEET.
The Liquid Bean Coffee Shop in Devil's Lake may be able to help you with bike problems if you need help (Owner's a big cyclist with lots of parts and tools). Other than that, no bike shops between Minot and Fargo.
At Island Park Cycles in Fargo, ND ask them for better directions to Pelican Rapids (I believe via CR 51 and 10, I could be wrong). They can help you avoid US 10. They have a cycling map for the Fargo area. This is for the Little Falls alternate.
Freewheeler in Minneapolis is the best bike shop ever (delivered a new wheel 200mi away).
Stop by the apple shop south of Lake City. The owner's pretty cool.
Uhm, I think thats it, so far.