In my house, we've been buying costco-brand (Kirkland) walnuts in mega-bulk from amazon for at least a year, and we've not yet encountered issues with their quality.
Trader Joe's has decent nuts, but their prices are not competitive when you buy in bulk.
Walnuts, in my opinion at least, don't vary in flavor that much (compared to almonds for example). So... walnuts are walnuts: always buy the cheapest! :)
But Trader Joe's usually has them too. Safeway sometimes does.
It's not that it's impossible, it's just that they don't have the same "staples" status they have in Europe. Usually, you find raw nuts in the cooking/baking ingredients area, while salted & roasted are considered "party food". And they're more likely to not have the party food variety than the raw variety.