how can i get visa to china i want to buy machine

If you have a supplier in PRC, you can ask him/her to issue an invitation letter and get the invitation letter of a duly authorized unit to you. And then you need to prepare the required documents including your passport and copy of its data page, the application form attached with a passport-style photo, invitation letter of a duly authorized unit, and an invitation letter from your supplier.
Afterwards, submit your application to the embassy of PRC to apply for a business M type to PRC to buy the machine.