It depends on what you're offering. If you're offering fully managed SEO services, and your work is done by small freelancers, then you should directly seek out companies and businesses. It could be by email, messages, physical letters, or even direct contact IRL. For this, you need to set up a proper business. You keep contacting businesses, keep getting work, keep hiring more people who do SEO. In this model, you don't do any SEO by yourself.
If you are just offering link building packages, PBN links, guest posting services etc. then just contact large to medium SEO companies who got the work and let them know your prices, what you can do for them and your TAT. Most SEO companies actually get the work from clients and then outsource it. So you do the work for them. And you need to be flexible.
Don't contact small SEO companies because they don't have much work and already have people on board. So, there is little chances they will hire you for anything.