It is so hard to find information about bus from Foz to Puerto Iguazu.
I finally found out that it is necessary to have something called carta verde to drive legally in Argentina (because of so many demands they have on that side).
Avis and Hertz are the safest places to rent if you want to drive to Puerto.
In case you want to take public transportation the bus from Foz is Crucero del Norte, and you catch in across the street from the Hotel Bourbon, and from the Puerto Iguazu bus terminal you catch a buz called El Prático.
If you can read portuguese, this blog is great for little tips on how to visit Puerto Iguazu: