Sorry I´m no expert on this, because I´ve only spent a couple of weeks in Iran, and carried a set of spare tyres with me.
But one of the reasons for doing so was warnings that I´d got, about the possibility of some rather heavy import taxes on goods, that are ordered to places like Iran & Pakistan. Not sure, if these were true, or if there could be a way around them, as I never needed to order anything.
Also didn´t look to me that there are many big bikes in Iran, +90% of bikes seemed to be 1-cylinder 4-stroke, looked to me like poor or quite decent copies of Honda or other Jap bikes, dozens of different "brands", with only the text on the tank being different! Their wheel sizes probably wouldn´t match a GS.
But if you absolutely need to find the tyres, Tehran could be the place to try, as its a huge city, and some expats there might have bigger bikes, and possibly better information about availability as well.