The only "bulk" seeds you're going to find in Seattle retail outlets are for things like ground covers and grains. Sky Nursery in North Seattle (it's
actually just over the border in Shoreline) has some
of those. Many online/mail order seed companies sell all kinds of vegetable seed in bulk, and of course the prices that way are
way lower than the single-packet prices. I would think Home Depot's prices to be about the lowest available for normal quantities of seed.