No one is ready to trust.

I am PHD student of Physics in Pakistan and want to go to china for the purchase of some chemicals that are necessary for my research. Its my first visit but no one is ready to trust me. The companies in china have sent me invitation letters but they are hesitant to provide me from foreign office. Because it needs a letter of guarantee and no one is ready to trust. The travel agents told me that tourist visa is closed for Pakistanis. The only way is business type that seems impossible now. If there is an other option plz tell me.......?????