I hate to tell you this, but there is nothing that will suit your needs. Western pond turtles are considered threatened or endangered in most of their range, is considered endangered in Oregon (so you won't be able to keep one there), and if you buy an adult, it is most likely wild-caught, which is an inhumane and ecologically harmful practice.
This article has some information on western pond turtles. http://www.natureali.com/pondturtle.htm
Also keep in mind that any pond you get will have to be escape-proof, or in an escape-proof yard, as turtles don't always stay put when relocated. If the pond is escape-proof, you'll either have to get a male, or provide some sort of laying area inside of the pond, as females can lay infertile eggs, even if there is no male. If there is no proper place for a female to lay, you could risk egg-binding, which can be fatal.
Western painted turtles might be native to your area, but I'd recommend finding a rescue or rehab to see if they get them in for adoption. Sometimes rehabs get in pets that cannot be released to native water ways for different reasons. The adult western painteds in stores are also wild-caught, so I discourage their purchase. You can visit www.anapsid.org to search for a rescue in your area.
Katrina