Whenever possible, try to get your icons as SVG or EPS files to ensure that they scale smoothly. Nothing worse than a pixelated icon!
Wikimedia Commons is usually a good place to look for things like this. You may not find everything, but you can find lots of major brand icons. H&M, for instance: https://commons.wikimedia.org/wiki/File:H%26M-Logo.svg
There are also loads of "icon providers" out there, many of whom have created icons for the bigger brands out there already. https://simpleicons.org/, for example, though that is only one of many possible sources.
Finally, you could also go straight to the brand's website. If they have a logo or icon in their website or app, odds are you will be able to find a decent icon there that you can save to a file. That's a bit more tedious, but still probably better than making your own.
- Josh Carlson
Kendall County GIS