Or, in this photo from an OGR thread, cjack's idea of placing a bridge rectifier in front of a DC-to-DC converter.
Depending on how much power you need, you might not even need an external capacitor...in which case you can get away without needing to fire up the soldering iron! That is, 2 of the 4 wire terminals of the bridge-rectifier go right into the screw-terminal connector of the DC-to-DC module. The calculation of a suitable capacitor value (if needed) is somewhat tedious - so best to confirm your application if considering this option. But in any case this alternative is also inexpensive.
