These are called half-covers.
When most military instruments were silver plated, these covers also saved some polishing.
As seen in this video during the opening turn into the road
http://www.youtube.com/watch?v=V8DXyAs_Ya4
the bass, euphonium, and cymbal players use sort of aprons attached to the front of their uniform jackets.
As for suppliers of half covers Phil Parker of London or McQueens of Manchester might be worth asking, if your tuba is of a British design.
For a more dogged protection of bandsmen you may watch the opening of this video:
http://www.youtube.com/watch?v=6uXdIaAOELE
Klaus