I have some military surplus pressure transducers that take 28 VDC at 75 mA max (measured current = 44 mA) and output 0 to 5 volts. I'd like to run them from some AA, C, or D batteries -- either 3 volts or 4.5 volts.

Can any of you folks recommend a particular DC-to-DC converter that would be suitable, along with a place to buy it?

 

Thats about 1.25W ... factor in even 80% eficiant conversion and you will be looking for circa 330mA out of your batteries at 4.5v ... they won't last long at that rate.

You cant do much about the power, it is what it is, but you can avoid the conversion losses and gain some additional use ....
Have you thought about using a string of 1.2V NiMh rechargables or possibly even LiPo cels.
High capacity AA's will get you the best mAh / £
Be carfull when buying ... a lot of C and D cells ar just AA's in tubes ... its cheaper to make them that way.

Al