I am located at Hong Kong so I can buy directly from Yong Heng's shop on Alibaba's e-commerce site in China. The advantage of buying direct from Yong Heng is that I can get very good support. E.g. the other day I asked them how the second-stage piston rings should be installed and they quickly took this video at the factory and send it to me via on-line chatting app :
https://www.youtube.com/watch?v=wgg8YouWZfs
This is definitely not what one can expect from resellers. For you guys in other countries, It's probably the best to buy from Yong Heng's official shop on Aliexpress.