Agency owner, ~10 years exp:

1. Talk to people. At the very start of an agency business, you get clients by talking to people - at networking events, conferences, via LinkedIn, etc. You don't get those first clients by sharing stuff on socials or doing paid ads.

2. You'll never make money if you start with that negative assumption - "companies that are strapped for cash already". Companies have cash, otherwise they wouldn't be alive. You just need to be a valuable place for them to invest that cash.

3. When you first start out as an agency, you'll do a lot of underpriced jobs, say yes to lots of things and learn a lot, but not make much money. Later you'll learn how to say no more and make more money. Read Blair Enns' book "Win without Pitching".