If you use Click & Pledge as your payment processor, you probably know they don't have the best online documentation. And they also create literally over 100 objects in SalesForce, making it nearly impossible to understand how everything flows and is related. However their support is actually pretty good. I called into their daily office hours to ask about source tracking, which it turns out has been in the system for many years now.
C&P Transactions in SalesForce have a field called Tracker. This field can be set by passing a value in a URL parameter named trk (it would have been better to call it tracker so that the labels matched up, but hey, that's just me). So if you have a link to an event you are managing with Click & Pledge, just add something like this to the URLs you create to your event:
And now you can filter and report on the Tracker field in SalesForce to see which lead source generated which admissions.