Post card material? You mean those cardboard type sheet used in printers to print I guess? Well the good news you there are tons of them from Epson, Canon, HP...to generic name ones too ( which can be cheaper and just as good) You can find glossy to matte finish too.
Just step into any computer shop selling printers like at Sim Lim Sq and I am sure you will find a section that sells papers and cards for the printer. And yes you can get that post card size ( and smaller) that's all ready to be place in the printer to be printed.
The bad news is....only when you don't have a edge to edge ink printing printer now or you have an old printer that does not have the proper setting to do printing on smaller size paper or card setting. So if you don't have that kind of printer you might have to invest in one. An edge to edge printed post card looking photo does look alot better