It is possible to get DC-DC converters with low noise, in the hundreds of uV (Linear Tech have some nice ICs) and can be followed by an linear regulator.
This doc has lots of tips to minimise switching noise.
I needed a negative supply for a project, and I'm using a charge pump method in an effort to keep harmonics noise power low, and low freq
ripple of a few hundreds of uV should be easily possible (I appreciate that may be too high for some scientific applications).
Personally I prefer to use dedicated and smaller supplies (switched, linear or combinations) where needed, and this allows me to power the entire
project from a single AC-DC converter power brick, I tend to have lots of 12V and 48V power bricks! My adjustable bench PSU sees little
use as a result. By dedicated I just mean a supply designed for the specific requirements in the cicuit, i.e. expected input voltage and
expected current output requirement.