where to buy Artix-7 or Zynq-7000 ?

Xilinx just started to ship the first samples of Zynq-7000 (see 
webside). You may not get the first "normal" devices before 2013, I 
think. Price depends a lot on the quantity, but you can read of $30 on 
the web if you really take a lot. Google for it.

If you need precise information contact xilinx or your distributor.