All four lamps on an S3 are 7-inch, and are easy to find. Holdens do a large range (
Holden | Electrical | Lighting | Light Units - 7" products), as do others. (Cibié are much cheaper from here for example:
Cibie Light Unit - 7" Round - Main/Dip without sidelights - Cibie H180 Light Units - UNBEATABLE PRICES! - larkspeed.com).
I'm told that the Cibié ones are better quality than some of the cheaper ones, with better lenses that give much sharper cutoff and better beam definition. I've just ordered a set of 5.75" Cibiés for the inner pair on my 410 to replace the sealed beams, so I shall report back once they're fitted.