Many (low quality) multi-voltage car adapters with a cpacity up to about 1A are nothing more than a bunch of series connected diodes with a switch to connect more or fewer in series with the load. The smaller the wanted output voltage, the more diodes (each drop about 0.8 to 1V). Better, but not great, stability compared to a resisitor which only works properly if the load doeasn't wary it's current consumption.

I'd look into building a small adjustable power supply. Not much more than an IC, a few caps and a potentiomenter these days...

/Michael