I've gotten Aisin and Denso stuff at Napa. My local O'Riely/Kragen has a really good guy behind the counter that has been helpful in the past but I only buy things like oil and paint from them (no parts except wiper blades). I feel my Napa has a good blend of help and parts. Might be worth checking out.
You're right about the tedious-ness of valve covers and injector installs, I did both but at different times. Not hard work but required a lot of steps and small parts.