In the biggest counties, here are the cities most commonly used for USDA financing:
Maltby (NE Woodinville)
Puyallup (limited areas)
Snohomish (east side of Hwy 9)
I did a very extensive list of all popular areas including some demographics of those ares on this blog post. I hope this is helpful.
USDA loans are for rural areas, so Seattle, Tacoma, Everett, Vancouver, Spokane and their immediate suburbs wouldn't qualify, but sometimes not too far off areas do.
I did a USDA loan for a client last year in Fall City WA, just east of Seattle about 30-40 minutes. If you want to let me know more specifically what part of the state you are considering I can give you more options or use the link below.