Where you go depends on what insurance you have if any.
Costco is the reference price for generic prescription drugs in Seattle. What this means is if you are persistant enough with some of the local pharmacies that advertise 90 day programs, you can get the same price as costco. The prices on this page are via costco mailorder but usually the local costco phamacy has the same price.
http://www.costco.com/Pharmacy/DrugInformation.aspx?p=1
Be aware that Costco does not bill medicaid and their medicare advantage program is closed to new patients. They have a limited number of insurance programs they bill. The pharmacies that match generic pricing usually accept medicaid and medicare (safeway, QFC and sometimes walgreens)There are probably others and there are specialty pharmacies around as well. Your insurance providers page will have a list.
Note: Safeway pharmacies have a good 90 day program and honor the costco generic cost lookup but I moved from there (roxbury) because they usually priced it wrong and I had to refuse and resubmit sometimes two or three times to get the originally quoted price. I use the qfc at 42nd now since I am on medicaid. That QFC is also one of the very few "compounding" pharmacies in town which means if your doctor prescribes two or more drugs mixed together, they can do it. Note the Costco on 4th ave is the number 1 store and the original (rebuilt on the same site) store of the chain.