ATMEGA328P-AU is a very popular and widely used Microcontroller.
Let's see the description of it as follow: AVR AVR® ATmega Microcontroller IC 8-Bit 20MHz 32KB (16K x 16) FLASH 32-TQFP (7x7)
You can buy a low cost yet new and original ATMEGA328P-AU from Cuocomponents, which is rated #5 in domestic market and ship the same day.