Since you also need a USB-to-serial converter, either of these boards include onboard 3.3 V regulated power output, probably with enough current to run a microcontroller:

https://www.sparkfun.com/products/11736

http://www.adafruit.com/products/284

In addition, this is an easy to apply 3.3 V regulator chip, in a package that's compatible with a breadboard:

https://www.adafruit.com/products/2166