Post cards, I got all of mine mainly from the MET in nyc but if you’ve got a local art museum they probably have some as well in their gift shop. Art museums usually only have postcards of what they’re presenting so if you don’t have Van Gogh nearby it’s best to look on amazon. You can also find whole sets of them on amazon for around 8 dollars. You can also just buy a Van Gogh book and rip out all the pages that have paintings on them.