I have used processors in both states.
In Washington, I have two choices. I can get a permit to slaughter them myself on the farm, then sell them from our farm within 48 hours of the processing. The second option is if I go to a state inspected procesor, I can then sell them off premises like at the farmer's market. The catch is we cannot process the birds at all, which means we cannot even freeze them. Since it's about a 90 minute drive to the nearest processor, I get as many people as I can to pre-order then pick the birds up at our farm. The balance I freeze them sell at the Farmer's Market the next Friday (and typically, chickens sell out within the first couple hours, even if I have 30 left to sell). My food safety inspector (the guy who inspects our farm) is OK with what we're doing, taking them out of state.
In Oregon it is my understanding that you must use a USDA processor for poultry to sell them off your premises (which really sucks). This may be wrong, but everyone I've talked to in Oregon has to skirt this law because it's entirely impractical.
The one I use most commonly is Harrington's in Boring, OR. The two nearest to me in Washington only do poultry 2 or 3 days per month, so it's an immense pain in the ass to work with them. Harrington's pretty much kills 5-6 days per week. You just have to be very careful to get your birds chilled to the right temperature for the ride home.
[edit] Also I forgot one thing. If you are doing other meat animals, Buxton Meats in Sandy, OR is just about 10 minutes past Boring. It is a small USDA inspected facility wihch means you can sell the meat retail once they have cut & wrapped it for you. It bears all the appropriate USDA approved labeling and everything. The one butcher is from Edinburgh, and trained in Scotland. He does a utterlay spectacular job on the lamb, beef and pig we've taken into him.