I bought my arduino from Partco Oy from Helsinki (in Finland). You can buy it from there too or fro example from amazon.com. There are thousands of arduino parts in amazon.com.
How mucht it cost? The smallest arduino cost’s only about 10 dollars, but the laargest arduino cost about 80 dollars.
Some of arduino parts can cost nearly 200 dollars like arduino 3G shield. But the cheapest arduino parts can cost less than 1 dollar.