How can I replace seal under the cistern on Roca Giralda toilet?

Try www.hotfrog.com.au
type in the search box top of page the following

' Toilet spares '

look down listing to find the title
Toilets, Equipment, Parts & Supplies - South Australia - Hot Frog click on that and look at the list. You might even be lucky enough to find one at your local home improvement store.
If it's like mine they are round firm foam with hole in middle of course and usually suitable for most toilet bowls/cisterns. Good luck.
Happy New year

Bob U.k
January 2013