http://www.bunnings.com.au/products_product_lamp-holder-fluoro-hpm-tombstone-type-screwd-t-cd380_P7053563.aspx
Yeah this is what I mean by tombstones, but I suspect yours ares screwed directly to the back of the backbox, this means you need specific ones that have provision for the screw attaching.
You might not find these at Bunnings.
Just take one of your original ones to an electrical retailer like Lawrence & Hanson or M&M electrical and they should be able to source them for you.
While your at it maybe replace the starter holder too, all these parts should be less than $50.
