I've had good luck matching o-rings at Napa in the past. But for injector o-rings I'm not sure... dealership would be your best bet by far if they have them in stock but would be the most expensive option.
Well Napa has the same Fel Pro FPG ES70600 that I got from o'reilly. They don't fit properly.
I used these o-rings from O'reilly and they worked fine. They felt a little loose compared to the originals but they don't leak. Maybe you got a faulty set?