Should have no problem with a big selling item like that.
OK, I'm being sarcastic.

In reality, 12" whitewall tyres were almost unheard of in Australia even back when our Datsuns were new. Yes, they did exist, but were never popular, so your best bet is most likely to buy a set of new tyres & get Mr Whitewall [or whoever] to do the whitewall job on them as several others have already done.

The only other way out is to specially order them [assuming that anybody actually makes them] & get them imported.