Is it possibile to buy online tickets for Beijing Zoo and Panda House? if yes, which is the website?

Yes, you can book the combo ticket for the two at the official website. The booking system only has Chinese version, so you can ask your Chinese friends for help. Or, just buy the ticket on the spot upon arrival. By the way, the combo ticket for the two costs CNY 19 from April to October; CNY 14 from November to March.