So, I'm a CPA. I worked as an accountant for many years and here are some of the things I had to do before I became partner (to be able to bring clients).

1-First understand the clients the CPA has today. You will find patterns and types of clients. You can get a correct knowledge of what kind of clients he/she is best at providing services.

2-Map all the networks his/her client base touches today. What companies do they do business with? There is no better referral for a CPA that an actual client. E.g: if he has one construction company, find who are the suppliers and other companies that work with this company. Know that you could approach them and have an instant referral (which is 80% of odds to get the client.

3-Which communities is he part of? Jews? Afroamericans? Latinos? Catholic? What sports he does?
There are networks in which we are involved that we don't realize that we can be helpful and find clients.

4-Who is the CPA that is the closest competitor? Find who his/her clients are? Which are you competitive advantages on comparison with them? Go for them. Approach just to ask what do they think about the service of the competition and ask what they would like to have that they don't. What other services?

Hope this get you started. Good luck!