My primary sources are:

• 7digital (who AIU primarily run white-label music sales platforms, but also have direct per-country shops like https://uk.7digital.com/). Reasonably simple interface, reasonable prices, good general coverage.
• hyperion records for everything classical.
• If everything else fails, Amazon Music; the way you get the music out of it varies over time, but in the end it has been DRM free for some time.