Try a caravan / trailer spares shop or similar
Most trailer spares shops will have one for about $80 to $100 brand new or you could get one second hand from a wrecker. Something to consider, if it has a significantly longer than original 50x50 section which causes it to go a long way out of the receiver, then the weight it can carry is reduced, due to the leverage. From recollection the trailer shop bloke I talked to indicated the extra 100 or 150mm on the one that I have to carry my bikes reduced the capacity down by about 800kg?
I bought a longer Hayman Reece one from Repco so I can carrying bikes without the bike carrier hitting the spare wheel. There would be a formulae somewhere to give you an indication of the loss of carrying capacity.