Aqua Forest in San Francisco is an ADA distributor, Seattle doesn't even have a major store that carries a decent selection of plants, never mind ADA stuff
There are loads of stores in the Seattle area with great plants and hard to find plants. The only place I know though to get ADA soil is The Wet Spot in Portland. I am surprised Robert didn't mention that. I purchased some Ludwigia sp. "Red", Hydroctyle tripartita, and Pogostemon erectus the other day at The Fish Store and some Penthorum sedoides, Pogostemon stellatus, and Limnophila repens 'mini' all at Denny's Pet World in Kirkland. I would say that is a fantastic selection of hard to find plant material available in the Seattle area!