Great question — answering as the UphillTech team:
Check out UTM parameters and start building them. You still can try going around it by looking at landing pages, referrers and trying to bind them to your transactions, but that requires a pretty good grasp on the data to properly drill in.
Full answer by BNazaruk: https://webmasters.stackexchange.com/a/140931 (CC BY-SA 4.0).
Need it done for your business? UphillTech.