In the form of your network, cold calls, cold emails, social networks and events (networking and Industry events).
Use your list of leads as the basis for reaching out. Ensure that the list includes a sufficent amount of prospects so that, when you get in the flow, you are not stopped in your tracks by running out of leads.
Think of this responsibility as creating opportunities to be rejected, and aim for being rejected daily.
If you're not being rejected daily, you're probably not proactive enough or learning fast enough with regards to what is and isn't working.