I use a 6 V, 3 A supply
that costs $7.68 on Amazon. I chose 6 V because this is the standard for bike dyno lights, and I really like the European bike headlights with the sharp horizontal cutoff and distributed beam. I use a Supernova E3 headlight and a B&M taillight, don't remember the model.
If you prefer 12 V, I don't have a specific recommendation, but see if you have any 12 V AC-DC converters lying around, or order an inexpensive one. Chances are they will work, especially with 20s or more. If it doesn't work on the first try, you might have to switch the input polarity.