Well, I did a quick ring around a few places in Hamilton. Most chemical places didn't want to sell anything less than 25kg bags at a time :-).
I found a garden center that actually had all the raw ingredients (although they called them funny names like potash, nitrate and magnesium ;-)). K[2]SO[4] was $3/kg, KNO[3] was $4/kg, and MgSO[4] was also $4/kg, so I got $20 worth of chemicals should keep me going for another few years...
Cheers,
Len.