OPGI can be a little higher on some things, but may have something that someone else doesn't.
My main sources are Ames and Performance years. The feedback in the Ames catalog is usually pretty helpful, but their shipping can sometimes be a bit high. The technical ability for the person on the other end of the phone there is not as good as it was years ago. Sometimes you can get someone that really knows these cars cold, other times the person doesn't have a clue.
Performance years is a bit more consistent and their shipping is usually a bit cheaper for me.
One might be a buck or two higher on one item, but that much cheaper on another.
I find that the quality of the parts seems to be about the same from almost everyone. They likely get the stuff from the same suppliers.
What I normally do is make a list of what I need to order, total it up from the catalog prices, and then go with whoever is cheaper.