We go over this every few months.
USA
In the US you can use
FindChips.com Search to search for parts.
First level suppliers are DigiKey.com, Mouser.com, Newark.com, and Allied.com.
Surplus places include Allelectronics.com, AllTronics.com,
**broken link removed** Jameco, JDR, and MPJA.
The are several places that buy and resell parts. Arrow and area51 come to mind. Expect to see a $50 min order from these places. Can be a last resort for obsolete parts.
International
Futurlec is sort of in a class by itself (so far as I know), cheap and slow.
For Microchip parts you are order from
Welcome to microchipDIRECT. I so nor know if Atmel has a similar site.