I purchased two boost converters or DC to DC power supplies, or whatever else people want to call them on eBay. One was a LiPower board from Sparkfun for $14.95. I assume it’s made in the US, has a battery connector and can be soldered with 0.1in pitch terminals. It’s setup for low voltage drop out at 2.6V but there is a small hack to reduce that (though I feel like you have better options if you’re trying two NiMH batteries because it has reduced current abilities from the hack.) See the sparkfun site for more details, including the link to the hardware hack. The output is a small solder jumper that comes preset to 5V but wouldn’t take but a minute to convert to 3.3V. https://www.sparkfun.com/products/10255
I put a small load (my anticipated load of a project) of 50mA and this device measured at 85.3% efficient.
The other device I bought was a cheap-o Chinese converter that came as a two-pack from Hong Kong. It came with a USB connector and took anything 2 to 5V. I tested this with a 2.4V battery pack and it seems to cut out at just under 2V. The efficiency was measured at 77.0%… Also with a 50mA load. My input voltage was a bench power supply set to 3.7V to simulate a LiPo battery on both tests. A nicety of the Chinese “mobile booster” board was the red LED showing output power status. The LED blinks rapidly when it goes into low voltage cutout. The cheap-o board also output 5.20v which was fine for my application. I think I gave roughly $4 both boards with free shipping.