No one seems to like the  imported power supplies that operate at a much higher frequency. But if you determine the frequency then you can build a proper filter et.
They are very efficient sometimes around 85-90 percent and can fit into a small enclosure

This type seems to be the survivor of the Chinese  DC-CD/Inverter/ZVS/Boost/Converter/Stepup  design competition:

https://www.ebay.com/itm/364620115446

There are lots of sellers of the same product, but with some slight variations.  All are based on the LT1243 or equiv. switchmode IC and all the parts and schematic are available (because I'm still ham-frugal enough to want to fix 'em even at this price!).    The output voltage and current specified cannot be reached at the same time,  so some skepticism is warranted.   For example, one I have rests at 313VDC and drops to 303VDC with 72mA load.  That's right at 20 watts so the regulation is still 3% - which is pretty good.   Since the sellers have no test data or even understand what the issue is,  the only way to learn their capabilities is "buy it and try it".  Some have a second output with negative voltage if needed.  Just understand they are non-isolated, i.e. the input and output commons are the same, so you can't do anything clever like stack them.

For some applications, the EMI they emit isn't a problem, and when it is, follow Breck's suggestion.  I had one light aircraft radio drive me nuts even so, but it made sense after I realized it had a 75 kHz IF - guess what the switching frequency of these power supplies is?   That one took a steel enclosure for magnetic shielding as well as electrical.

73, Bob W9RAN