In Marketing Cloud, there’s not really a good way of finding the original data source for a subscriber in a send — especially if you’re sending to multiple data sources for every send.
There is, however, a personalization string called _DataSourceName, outlined on the Data About the Recipient section of the Personalization Strings page in the documentation.
If you’d like to record this data, you can simply add an AMPscript variable in your email or template and set it to _DataSourceName. If your Send Log Data Extension has a column matching the variable name, it’ll get recorded for all of your sends. (Fields that start with an underscore aren’t allowed in Data Extensions, so you can’t just name it the same as the personalization string in order to record it.)
Send Log Data Extension:
If I do a single User-Initiated Send to multiple Data Extensions (DataExtension1 and DataExtension2), the following is written to the Send Log:
(I removed ListID, BatchID, SubID, TriggeredSendID, ErrorCode and SendDate columns from the above table for clarity)
Once you have this data in your Send Log Data Extension, you can correlate it with other data using a Query Activity or export for use outside Marketing Cloud.