The good thing about a young franchise is that history is short and the number of trades is easy to track. Because of that I have started to register all trades the Diamondbacks have made with each team and see if anything interesting pops up. It is pretty easy to find the trade history of the Diamondbacks, because Baseball Reference makes it accessible.
If you look at that page you will see that the D-Backs’ most popular trade partners are from the American League: New York Yankees and Chicago White Sox (16 trades with each team).