third location sync

[Paonia Ezrine]

Can this be done with rsync?

I have two machines, (a) and (b).
(a)= has a fast connection
(b)= has a very slow connection
Both have a directory called /debian which on day one is identical.

The copy of /debian on (a) is synced weekly with a debian site.

On day 30 (a)'s and (b)'s copies of /debian are quite different.

The owner of (a) will put any files I want on a cd and mail it to me.

I would like run rsync on (a) and reference the files on (b) and create a
/debian-move directory on (a) that only contains the files which are in (a)'s
/debian directory, but not on (b)'s. Is this possible with rsync?

I have figured out the following, not very straightforward, way of doing this:
1. run a find on /debian on (a) to find all the directories
2. create the directories for /debian-move based on this information
3. run rsync in dry run mode
4. massage this data to figure what needs to be copied
5. copy these files

Is there a better way?

Thanks,
Paonia