Well, here's the thing:
If you go to virtually any bunnings in Australia, you'll find grafted lemonade trees for sale. Literally.
Millet is offering Lemonade seeds in the citrus buy & Trade which are going begging. These will be the same as what is available in Australia.
However, you will almost certainly never find them for sale in stores anywhere. In fact, I've never seen them sold in stores.
One place I would definitely recommend your friend visits whilst in in Brisbane is Tropical fruit world; here. he can sample a range of exotic fruits, including the lemonade. It's probably an hour or so drive from Brisbane, but you get there by the highway so it's an easy drive.
http://www.tropicalfruitworld.com.au/