so how would one go about joining tables from multiple data sources without database links

One wouldn't!

If you have multiple data sources which are in different databases, then you need to link to them somehow. Data cannot flow from one place to another without some form of link...

and without temp tables...

Why would you want to???
The creators of RDBMSs and languages give us the facilities to solve problems. It is up to us to use them in a sensible way. Choosing not to use one or more is a bit like trying to cross a river by paddling when a perfectly good boat is ready and available.

I know of no modern RDBMS that does not support temporary tables.