Funding sources can be easily added by the network admin.
If you are a network admin, simply click on your [Admin] tab and do the following:
Click on "Edit Network Settings" link on the left side of your screen
Click on the [Funding Sources] tab
Click "add funding source"
Enter the name of your funding source
Click [Save Changes] see screenshot - https://simon.d.pr/96xmMn

When adding assistance, you will find the "Edit Funding Source" link towards the bottom of the "Add Assistance" window.  Click on that link to access the drop-down options and select the appropriate funding source.  see screenshot - https://simon.d.pr/Hpf3uR