I've read that I should be careful to use a high-quality grout sealant for my shower tiles, not the stuff they sell at Home Depot. What brands would be best, and where would I buy them (in the Seattle area)?
I've had good luck with StoneTec stuff. You might have good luck contacting a granite fabricator - they can't afford to use lousy stuff. I mail-oredered mine from the TYW store on www.johnbridge.com. Normally, you don't need much. The really good stuff is quite expensive, but not bad if you can find it in say pint bottles. The good stuff can cost in the order of $150/gallon, but that is WAY more than a person would need for a home - that would last probably 200-years! The stuff generally goes a long ways.
I've used aquamix sealer on recommendation from john bridge too. It worked great! I found it at a local hardware store. Another place to try is looking up tile stores in the yellow pages or on google local they usually have the better stuff.
Tile For Less in seattle or bellevue sells AquaMix brand sealer - I've used it a few times on showers and it works well. I can also recommend StoneTech - I've used their granite sealer and it's great.