Use the search term “AC-DC to DC buck converters” in ebay for example. You’ll get a lot of hits. Pick the one you like. As Mike Mottler says a used PC power supply can be had for cheap or maybe free if you have an old computer laying around. I use one of those for a bench supply. It has taps for 12, 5, and 3vdc.
This DIY device was designed by OGR member Rod Stewart for a small dc power supply on our layouts. Pictured is the narrow version. The write up is in the forum electrical-reference-materials-andamp-manuals