Ivry, I don't think there are any vans on straight floats in Australia
As far as I know you've got three choices in Australia.
Sea Air at the Gold Coast do Caravan endorsements but I'm not sure if they've got a van on floats at the moment (last I heard it was on wheels), but worthwhile giving them a call
Training.
Sydney Seaplanes have a good fleet of vans and Beavers.
Float Training
Last, but not least, Air Whitsundays also does endorsements
Air Whitsunday Seaplanes | Seaplane Endorsements & Seaplane Advanced Floatplane Training