How do you obtain official brand logos for design prototypes?

I need to use company logos as a part of my designs. These companies are institutions that are stakeholders for my client. Some sites have SVGs you can grab from the website but how does one go about getting those that are not available on the site in a useable format like a PNG or SVG?

Should I ask my client to ask these companies for design assets? Does my client need permission for these logos as well?