Try http://www.prairiemoon.com
You should be able to find some suitable mixes there.

I must first say I am not an expert, just have some experience. Sounds like you are on the right track. Just do some research on your local genotypes and follow the usual good advice and common principles. I would look around for natives observed in your region, give consideration to soil type, exposure, moisture levels and...be patient ;). Take your time and do the research for success. You may have some local native prairie "afficionadoes" available. Check them. Best regards. Mike in Indiana

If I was doing this project I wouldn't bring in topsoil, unless you really don't have any soil at all, which is probably not the case. Imported topsoil is likely to have lots of unwanted weed seeds, and is not necessary. There are native wildflowers that will thrive in any type of soil, and you'll often find that rich soils benefit weeds more than they benefit wildflowers and native grasses.

You might not have native prairies in your area, but you do have native grasses and wildflowers which form native meadows in open areas. I think these open areas should be the model for your planted prairie. Look both regionally and locally for examples of native plant communities that are stable at least for a few years. You will probably have to always remove some none-native species and woody species, but I am sure it is possible to create a relatively stable, attractive native meadow in your location.

I, too, live in the Pacific Northwest, specifically in North Bend, WA. I have about 3500 sq ft that is slightly sloped where I am going to create a wildflower meadow. It originally was covered with weeds, grass and some blackberry bushes. First we killed it several times last summer with weed killer. Then we rotor tilled the entire area in September, 06' and covered it with black plastic all winter. We have not and are not planning to add any top soil and there is not an automatic irrigation system in that area. It is in full sun and on our septic drainfield.

My plan is to purchase about 5 lbs of a "native" wildflower mix. Rake the top of the dirt to loosen it, mix the seed with sand to make it easier to distribute evenly and then after distributing it, lightly rake it to mix it into the dirt. Then, cover the area lightly with straw so it doesn't get washed away by the rain.

My understanding is that for the first couple of years I will need to weed out the larger weeds before they go to seed and run a sprinkler on it during the drought summer months. Then (hopefully) by the 3rd year the perinnials will be large enough and established so that they will not need too much additional water even during the dry summer. And (again hopefully) the weeds won't have prevented the wildflowers to fill in. I know that I will always have to weed out the larger weeds and will have to add additional wildflower seed to fill in patches that are not taking well, but my plan is that it will be eventually be a low maintenance pretty area with lots of color. It is on one side of my driveway that is at the street level, so you can see it looking at my house from the street.

I'm anxious to hear how yours comes out.

spikelet,
Consider contacting your local field office of the Dept of Ag/NRCS. http://www.nrcs.usda.gov/. They provide free consultation, design, among many other things. Also, look at the programs. You may even qualify for one that pays for part of your conservation project.

contact your native plant society...they should be able to help you out.

DONT bring in top soil. rich soil will just encourage weeds...plus most natives do fine on poor soil.

also visit native prairies in your area. also there are good books on prairies...see my website.

Here is a link that might be useful: my garden