Revegetating a steep slope can be a difficult task in areas of frequent rain such as King County. You should cousult an excellent web site on the Internet that deals with this problem in Washington and offers many suggestions, including suitable plant species for use on slopes. The species recommended are mainly trees and perennials. It would be good to establish deep-rooted plants if there is danger that shallow failure planes in the soil might trigger down-slope soil movements (landslides) in wet weather. However, the best plants for preventing erosion are grasses because of their extensive fibrous root systems. Here is an excerpt from a previous Mr. Smarty Plants answer that listed grasses native to King County, Washington that should work well for your project.
Agrostis exarata (Spike bentgrass)
Bromus carinatus (California brome)
Danthonia californica (California oatgrass)
Deschampsia cespitosa (Tufted hairgrass)
Dichanthelium acuminatum var. fasciculatum (Rosette grass)
Elymus glaucus (Blue wild rye) — especially recommended for erosion control
Festuca occidentalis (Western fescue)
Koeleria macrantha (Prairie junegrass)
A further description of suitable grasses is found at this web site. Mr. Smarty Plants recommends that you use one of the native species. The cheapest way to carry out your project is by sowing grass seeds. However, since you would be sowing the seeds on a hillside, rain is likely to wash the seeds away before they have had a chance to germinate if the slope is severe. You might be able to find grass plugs available for sale. They will be more expensive than the seeds but would have a better chance of setting their roots before rain could wash them away. Another (alas, more expensive) possibility is to use erosion control blankets to stabilize the erosion area. The erosion-control fabric works by slowing the runoff water and allowing sediments to fall out rather than be washed away. Seeds are sown under the erosion-control material and grow up through the matting when they germinate. You can insert plants into the soil by cutting through the matting. The roots of the plants that are growing through the erosion-control material anchor the soil to stop the erosion. If you use erosion-control blankets made of biodegrable material, they will eventually disappear leaving the plants to control the problem. Matting is commercially available, for example, at Native American Seeds.
You can check our National Suppliers Directory for possible sources in your area for trees, forbs, grasses and grass seeds.