You can usually get replacement sprues direct from the companies. I have in the past for 1/35 from DML with no problem, and Italeri, but they required me to send them part of the box to prove I had the kit and then went through the distributor, so it took a while which sucked because it was preordered MTB and the part missing form the box was a transom.
I would try Trumpeter, but I think I've heard in the past that they were hard to contact.
By the way, before the AAF museum started carrying them, Tamiya was the worst for this. I had to shell out $50 for the Tiger I cupola years ago. I think I got the entire M4 turret for $12 from the museum.