hello, how can i get invitation letter for applying tourist visa?

Hey, the invitation letter can be issued by a relevant entity or individual in PRC. If you have friends in PRC, you can ask them to issue it to you. Or, if you have the return air ticket booking record and proof of a hotel reservation, you don’t need to get an invitation letter.

Is this posible that i can travel to Xiamen without invitation letter if I have a return ticket and hotel booking reservation? I just want to travel to Xiamen 3 to 5 days..

Yes, it's ok. You don't have to provide the invitation letter.