Believe it or not....Pep Boys!
You might ask for a special order.
They didn't have any on the shelf and I told them that I needed 6 cans. I had them the following week.
The Home Depot has white primer too.
I've used the Rustoleum white primer from The Home Depot and had good results. It dries fast too, not like the rustoleum paints themselves.
I think that Coverite sells a white primer. That gets expensive though. Hobby products seem to carry a higher price.