Could you go onto the CBBC website and print off lots of pictures of the Octonauts and then glue them onto paper plates and the party bags? There are lots of colouring pages on there too so you could include a print out of them in the party bags with some crayons for little ones.
My DS1 is an octo-nutter too so I feel your pain. I really havent seen any plates etc in production - but do know Amazon are now selling octonaut toys..
Was going to suggest same as Dinky - CBeebies have loads of great stuff to printout. If they have to colour and stick it will give them something to do as well. Always nice if they can make their own custom-cup