Go to the Microchip product page:
and select "Show all products" (if not only New/Popular items will be displayed).
And click on the Product Search tab you'll find the microcontroller selector. Using the scrollbar within the page (it's in an odd spot) scroll across to UART and pull the slider down to one and that will show the ATtiny devices with a UART (currently 7 devices).
Most manufacturers and some suppliers have similar search facilities which are normally called a parametric search. Also as mentioned in some comments you can also look at a software / bit-banged serial interface if suitable for your project and then just about any microcontroller will do. That's just implementing the serial protocol in software using regular I/O pins.