Find SEO/SEM companies and partner with them. They have a need for your services for their clients. You can offer them a small percentage of your monthly revenue for each client they bring to you.

Do the same with web development firms. For direct outreach get on LinkedIn and search for small to mid-sized companies and get in touch with the marketing VP or directory and offer your service to them directly as well.

Simultaneously, you should also start actively employing SEO on your site so that you can eventually rank for keywords related to social media marketing.

People are already actively looking for what you are doing and being found on page 1 for an organic search is going to bring you all the leads you can handle. That's a long term strategy though.

Try the JV/Partnership route first and leverage other people's networks for your services. Most, if not all of this requires you do nothing but email and maybe a few phone calls to close a partnership deal.

If you have a budget you can also run ads on LinkedIn and Facebook but first, consult with someone who can show you how to do that the right way without spending a ton of money for no results which is incredibly easy to do :)