Ask a local garden club.

Check Craigslist and other classifieds.

Ask nurseries and landscape contractors.

Check with the City - do they have yard waste mulching/composting facilities?

Much is typically made from locally available materials - in the PNW, that means Doug fir/hemlock/cedar, sometimes alder. Seldom pine. Pacific Topsoils probably has the largest selection of available mulches and they have multiple soil yards in the Seattle area. Pacific Garden mulch is my favorite.

Here is a link that might be useful: Pacific Topsoils mulch products

Garden centers and the big box stores have limited mulches for sale. You may find a wider variety at a landscape suppplier, such as gardengal has suggested.
Keep in mind that a mulch can be any vegetative waste material, not just chipped wood.

arborists and tree trimming services are a good source of cheap mulch.