Hi Ped,
I get lead balls from a company here called "Clay and Game", they do quite a selection of lead shot, you do have to order min. 5 kg, last time I bought 4.5 lead balls which would have cost me approx. 4 times more if I bought the Gamo type, They have No 4 shot, but I don't know if that is the same as No. 4 buckshot, worth a look.
Cheers,
Lawrie.