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!
I have to agree with Mary, Seattle area LFS have some great plants if you know where to look. Occasionally A Place for Pets in Burien has Java Fern 'Trident' and Pogostemon erectus. At Sierra Fish and Pets in Renton they regularly have Nymphoides sp 'Taiwan' and occasionally they have Hygrophila pinnatifida.
Robert may have to make a trip up to Seattle and do some scouting; the selection of aquatic plants in Seattle has changed substantially in the last few years!
Roy 75 Gallon, 2X55W AH Supply CF 8800K, 1XFluval F&P 2.0 48"/59W, 2X Marineland 350 Magnum filters; 45 Gallon Tall, 96Watt AH Supply CF 6700K, 1X Marineland 350 Magnum filter; 30 Gallon Long; Fluval F&P 2.0 36"/46W, Marineland C-220 filter; 20 Gallon, 1X26W AH Supply LED kit, Marineland C-160 filter; all with Press. CO2 and (Calcined) Montmorillonite Clay Greater Seattle Aquarium Society (GSAS)