can i buy more memory for my macbook air?

you cant, no. Its permanent on the logic board.

You can only BUY more RAM when purchasing / ordering the computer,

it cant be added later on by anyone.

No, the Air cannot be upgraded, the memory is soldered to the logic board.

Perhaps you mean storage. If so there may be a possibility. It will depend upon your MBA. Examine this link:

http://eshop.macsales.com/shop/SSD/OWC/Air-Retina

Ciao.