Find 1st SFDC Campaign Value?

Do you have a value in the Acquisition Program field?  If so, that should work.

Thanks. I noticed that I only have 5k leads w/ acquisition programs. How does this field work exactly?

The Acquisition Program (and Acquisition Date) is set automatically by Marketo when a new lead is created as a result of a Marketo program.  For those leads that were created before you implemented Marketo programs, you can export a list from Marketo that has the SFDC Campaigns column, but it includes all the campaigns they are a member of, not just the first one.  You  may be able to use fields like Original Referrer, Original Search Engine, Original Source Type, Registration Source type and Registration Source Info to help identify the lead source.  Another possibility is to export your leads with their SFDC ID and creation date, export the campaign member reports with their SFDC Lead / Contact ID, campaign name/ID and campaign member response date and then use Excel's VLOOKUP to cross-reference where the campaign member response date and the creation date match.