I'm not the only one here that'll say Plastikote T-237 is the best out there. It can be difficult to find, however. I can order it from a supplier at work so I'm fortunate. In fact, I have used all four of the available colors and all of them are truly excellent. Plastikote makes another white sandable primer that comes in a can like this:
It is a good primer too, but it isn't the same. I can't complain about it, but it doesn't seem to cover as well as the T-237 does. I'd be willing to bet the stores that carry Plastikote(besides craft stores) can probably order it from their supplier if one were to ask the manager about it.