Hi Paul, Dripworks is the best place to go for low pressure drip and microspray irrigation supplies. They are in California and the shipping is really quick. They also provide design advice.
If you have a business license they will sell to you wholesale, with a generous discount.
You may check out Steuber's in the town of Snohomish; they have gobs of irrigation stuff. They don't have a website.
Loads of drip irrigation is available from the Ace on 90th and Roosevelt (if they don't have it, they can order it!) or from where I work, Greenwood Hardware. You can also order anything we don't have on hand through the True Value website and have it delivered free to the store.