The Pololu 5V Boost Regulator NCP1402 is a very tiny source of 5V from supply voltages as low as 0.8V.
Need a tiny source of 5V, and only have space for a single battery? You need a Boost Regulator. These devices take a low voltage, and convert them into higher voltage (at the expense of more current draw - no such thing as a free lunch, with all that entropy going on!).
The Pololu 5V Boost Regulator NCP1402 generates a 5V power source at up to 200mA from supply voltages as low as 0.8V (like an almost-dead AA battery!). Based on the NCP1402 boost regulator chip from ON Semiconductor, this tiny package can sit on a dime and power your circuit!
And it's inexpensive too!