You won't believe this Bob, I have been chasing 7.50-16's for a couple of months, and only managed to find a set of 9.00-16's at Wagga Swap last month.
I assume you have tried Morree Ag tyres?
(Shortage of lugged 7-50's is apparantly due to the earthquakes etc overseas, all the good tyres are in demand, AAustralia is low on the list....Have had to buy ex army tyres.
Also, see Antique Tyres in Melbourne, they list Bar treads:
http://www.antiquetyres.com.au/16900
Rich.