If it's a hobby — look at the side of the Arduino and its analogues. For a start.
If you decide to engage in professional electronics development — choose the target system with the right peripherals you...
Well, where to buy — you decide. There are a lot of factors — the cost and availability.
For example, Muscovites and Petersburgers — Arduino and analogues can be found in stock in shops (Chip and Dip, Linuxcenter). But there will be more expensive than over the Internet.
Using the Internet is cheaper but there is shipping time. You can buy through Ebay or other Western sites of the original Arduino. It is important to remember about shipping, unfortunately, it's not the fastest option.
Another option is Russian counterparts, they are not very much, but there is a choice. Here is the best offer on cost and shipping. Manufacturers freeduino.ru robocraft.ru jt5.ru
There are also shops — ampere and other, rock search.