A referral is a trusted party referring another trusted party, for example a friend introducing a friend.
This trusted party might be someone you worked with previously (see #1) or someone who knows and trusts you.
Referrals don’t happen often unless you ask for them, and how you ask for them is important. Remember, as a consultant you exist to provide value to clients.
For example, in existing clients, you can ask “Is there anyone else you could introduce me to that I could provide value for?” or “Is there anywhere else in the organization I could provide value?” Doesn’t that sound better and more professional than, “Do you know anyone else who might want to pay me?” or “Who can you introduce me to that might have work for me?”
In most of my large clients I have consulted for multiple people within the organization due to referrals.
Also, if someone cannot refer you within their organization, they may be able to refer you to someone who may be able to use your services in another (usually non-competing) organization.
Assuming you do work in your local community, you can ask your accountant, doctor, attorney etc. for referrals whenever you normally interact with them.
Lots of people can potentially provide referrals. I’ve even had high school friends and friends I’ve met through common hobbies refer me.