Ok, done a little research.

There are baking and grilling plans.

Baking planks are reusable and should last forever.

Grilling plans are a use-once type planks. Trey are relatively inexpensive. Not as cheap as going to the mill, but hey, it's not like mills are plentiful in the greater Seattle are.

For those not wanting to drive out to their local mill, I suggest one of the Made in Washington stores.

A 4-pack of grilling planks is $7.

The banking plans are $39/$49 (ouch).