The Arduino Uno MCU is fairly widely available: https://octopart.com/search?currency=USD&q=Atmega328p&specs=...

although there are others MCU's with even better availability (e.g. the SAMD21, which also has plenty of dev boards and it seems like Microchip has extensive stock on hand, for example https://www.microchipdirect.com/product/ATSAMD21J17A-MUT?pro... ). One can take any of the devboards a a reference design.