White primer should be commonly available. I'm sure Krylon has it, if not I have sucessfully used white body primer that you buy in any automotive parts store, i.e.: Pep Boys, ect.
I found that these sandable primers have a good "bite", sand and cover great and really have worked flawlessly on all my projects in the past, including my Fett project.
Hope that gives you a lead!
I usually just go down to Pep Boys and get their white sandable auto body primer. It has worked on everything so far and I also never had a problem using a different brand paint over it.
The only time I had a problem was once on my armor when I sprayed my next color using Krylon over Rustoleum... that didn't work well (paint cracked and lifted right off) so I had to put a clear coat inbetween. But for the primer.... never had a problem yet.
I had really good luck w/ testors primer. It sticks very well & worked great even on my 96 DP! It does come in a variety of greys and white as well. It is a little more spendy than krylon though. Just an idea
Krylon and Rust-o-leum American Accents both make white primers.. I have seen them both recently at my local Walmart, however they are not always in stock so you may have to look hard for them.. The auto-body primer sounds like a good bet though.