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:
If you're interested in a higher-power version, I just used the SC189 3.3V switching power supply in a design of ours. It can supply 1.5A (provided of course your upstream power can too).
Others are suggesting power supplies, but I think you can just use a voltage divider from a 5V supply. Like maybe a 2.7K resistor followed by a 5.6K to ground?
It needs to be regulated, because the microcontroller draws varying amounts of current. But a simple regulator is just a three-legged IC and a capacitor.
The Rload and R2 are parallel resistors really so if Rload is really low which it is generally if it's doing anything then it will effectively shrink the R2 resistance (Rload || R2) and therefore voltage. Then whatever is connected as Rload won't get enough volts + current.
You can fix this to some degree by using very low values of R1 and R2 but then they start to sink lots of current as they are in series across the power supply and get hot so this becomes an efficiency problem. If you're really unlucky the magic smoke comes out or you touch one and burn your fingers emitting a nice bacon smell.
You can fix all of this with a single NPN transistor and a zener diode pretty easily but you might as well use a regulator IC then.
Analogue electronics is fun even if it does burn and explode in your face occasionally! My shit blows up all the time: http://i.imgur.com/GZBKblt.jpg
