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! :)