In fact the US federal government could be your largest potential customer and, by law, must do business with small businesses. There is no shortage of red tape to get there but the rewards, and revenue, your small business can garner may be worth the effort.
Read more from the SBA to get the basic info and see if it makes sense to call Uncle Sam your client.
https://www.sba.gov/blogs/does-your-small-business-qualify