how do i get an invitation letter from a chinese company

You have to have cooperation with the Chinese company, and then you can ask them to provide you an invitation.

As for the visa notice, from January 1st, 2011, it has been substituted by the "Invitation Letter of Duly Authorized Unit" or "Confirmation Letter of Invitation". It is a letter issued by an authorized unit in China, such as Chinese government departments, companies and social organizations authorized by the Chinese Foreign Ministry. The letter should be enclosed with the full name and passport number of the invitee as well as the purpose and duration of the visit, and should be used within 6 months since its issuing date.