Trumpeter has just launched a Part Replacement Service, which can be used to order individual sprues.

The new service is accessible here:

http://part.trumpeterkit.com/

For a test I tried searching 05553 and takes you to a list of sprues that you can order through Aliexpress.

They are still adding items to the database so not models are currently available.

It is good to have now a straightforward way to order a replacement if you need additional sprues of a kit for any reason