I am an American Citizen living in Dushanbe. How do I obtain a tourist visa?

I am in Dushanbe with my husband who works for Save the Children. I want to visit some British friends in Beijing for 8 days at the end of October. I was told I need an invitation letter, original, from any travel agency. On this website there is no mention of an invitation letter required for a TOURIST one. I would be a tourist only, not a student or journalist. Can I get it? I look forward to your response. Best Regards,