Many domestic oil cylinders are mainly used in construction machinery, so many construction machinery manufacturers have their own oil cylinder factories:

For example, Xugong Hydraulic Parts, Longgong Oil Cylinder, Xiagong Yin Hua, Sany Zhongxing and Zhonglian Changde. Do all cylinders.

There are also many other manufacturers specializing in oil cylinders: Jiangsu, Zhejiang and Guangzhou.

For example: Jiangsu Hengli Oil Cylinder, Guangzhou Shaoguan Weiguang, Changzhou Rexroth, Tianjin Pike, etc. You can search online, there are many manufacturers.