I’m looking to print postcards. In the US this was super simple with an online service like Vistaprint. Anything like that in China? (I don’t mind if it’s in Chinese, as I would expect it to be) Or perhaps a good brick and mortar store locally that could do it?
Look for any printer shop. they have a wide range of services up to A0 photocopying/plan copying (B&W) I have had A3 color printing but I am sure then go much larger posters. Most do business cards, and the main difference with what you want, would be if you want photo quality paper. I would not be surprised if many of these shops do.
If you are new to China, these shops are easily spotted as they usually have an A4 piece of paper the word 'CAD' in the window, or stuck on the door. Some of these places are not much more than a hole in wall, but they know their stuff. Try a couple of these places first and you should have the answer as to whether or not they can do it.
I just entered 印刷服务 (printing service) into Taobao and got a lot of hits. Many do glossy booklets/company catalogs. The covers of which are similar to postcard material. They probably do many grades of card. It is worth a punt.
Could also search for 定制明信片